modified hydraulic clam digger · 2013-02-26 · to maritime clam flats. these include: (1) a...

f Environment Canada Environnement Canada

Fisheries and Service des peches Marine Service et des sciences de la mer

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Digging Efficiency Trials With a Modified Hydraulic Clam Digger

by

D. W. Smith and H. LeBlanc

Technical Report Series No. MAR/T-76-1

Resource Development Branch Maritimes Region

t

DIGGING EFFICIENCY TRIALS

WITH A MODIFIED HYDRAULIC CLAM DIGGER

D.W. SMITH AND H. LEBLANC

MARCH, 1976

TECHNICAL REPORT SERIES NO. MAR/T-76-1

RESOURCE DEVELOPMENT BRANCH FISHERIES AND MARINE SERVICE DEPARTMENT OF THE ENVIRONMENT

HALIFAX, NOVA SCOTIA

iii

CONTENTS

LIST OF TABLES

LIST OF ILLUSTRATIONS vii

ABSTRACT ix

INTRODUCTION 1

DESCRIPTION AND OPERATION OF EQUIPMENT 2

METHODS 4

RESULTS 6 Population Structure of Coke Flat 6 Population Structure of Baker's Island 6 Efficiency of Hydraulic Digger 9

Effect of Digging Speed on Harvesting Efficiency. . . 9 Effect of Nozzle Angle on Harvesting Efficiency . . . 10 Effect of Nozzle Height on Harvesting Efficiency. . . 13

Breakage and Condition of Clams 18 Problems Associated with Improper Use of the Digger. .. . 18 General Observations 19

POTENTIAL IMPROVEMENTS TO DIGGER 21

SUMMARY 21

ACKNOWLEDGEMENTS 23

REFERENCES 25

LIST OF TABLES

TABLE 1. Soft-shell clam harvest and extent of breakage during hydraulic digging trials on Coke Flat, Halifax County, 1973 7

TABLE 2. Soft-shell clam harvest and extent of breakage during hydraulic digging trials on Baker's Island, Halifax County, 1973 8

TABLE 3. Harvesting efficiency of hydraulic digger operated at different speeds on Coke Flat, Halifax County, 1973 11

TABLE 4. Harvesting efficiency of hydraulic digger operated at different speeds on Baker's Island, Halifax County, 1973 12

TABLE 5. Harvesting efficiency of hydraulic digger operated at different nozzle angle adjustments on Coke Flat, Halifax County, 1973 14

TABLE 6. Harvesting efficiency of hydraulic digger operated at different nozzle angle adjustments on Baker's Island, Halifax County, 1973 15

TABLE 7. Harvesting efficiency of hydraulic digger operated at different nozzle heights on Coke Flat, Halifax County, 1973 16

TABLE 8. Harvesting efficiency of hydraulic digger operated at different nozzle heights on Baker's Island, Halifax County, 1973 17

TABLE 9. Comparison of hydraulic clam harvester production at different digging speeds, on Coke Flat and Baker's Island, Halifax County, 1973 20

V

LIST OF ILLUSTRATIONS

FIG. 1. Landings and value of the soft-shell clam fishery (mya arenaria) in the Maritimes, 1947-1972

FIG. 2. Perspective drawing of experimental hydraulic clam digger 3

FIG. 3. Map of Musquodoboit Harbour area, Nova Scotia, showing experimental clam digging sites at Coke Flat and Baker's Island 4

FIG. 4. Population structure of soft-shell clams sampled on Coke Flat, Halifax County, in 1973

FIG. 5. Population structure of soft-shell clams sampled on Baker's Island, Halifax County, in 1973 9

FIG. 6. Harvesting efficiency of experimental hydraulic clam digger during 1973 trials at Coke Flat and Baker's Island, Nova Scotia . . . . 10

vii

1

6

ix

ABSTRACT

A modified Price-Bourne hydraulic clam digger was used on intertidal beaches to determine its harvesting efficiency and to assess the resulting damage to the clams and the beach. Performance tests were carried out on two types of beach substrate, hard mud-sand and muddy-sand, which were considered typical of Maritime clam flats. The digger was operated at five different speeds, ranging from five to twenty feet per minute. The water pressure at the manifold was maintained at twenty-five to thirty pounds per square inch.

During operational trials, the hydraulic clam digger demonstrated a two- to five-fold increase in production per tide over hand digging, required less physical effort, and harvested up to ninety-five percent of the clams in its path. More important for conservation and management, breakage amounted to less than three percent of the clams dug. There was less than five percent mortality and little damage to undersized clams left on the beach by the digger.

RESUME

Une machine hydraulique a Ocher les coques Price-Bourne, modifi6e, a 6t6 utilisee sur les plages intertidales afin d'en determiner 11 efficacit6 et dievaluer les degfts causes aux coques et a la plage. Les essais ont 4t6 effectues sur deux genres de substrat de plage, soit de vase dure et de sable, soit de vase et de sable, consid6r6s comme etant representatifs des platins a coques des Maritimes. La machine a fonctionn4 a cinq diftdrentes vitesses allant de cinq a vingt pieds a la minute. La pression d'eau au tuyautage diediappement a ete maintenue entre vingt-cinq et trente livres par pouce carre.

Au cours des essais, la machine hydraulique a Ocher les coques a augment6 de deux a cinq fois la production par maree par rapport A. la Oche a la main, a .exig6 moins d'effort physique, et a permis de recolter jusqu'a quatre-vingt-quinze pour cent des coques sur sa voie. Les coques cassees representaient moins de trois pour cent des coques pechees, ce qui est tres important pour la conservation et la gestion. La mortalit4 s'elevait moins de cinq pour cent et les coques trop petites laiss6es sur la plage par la machine avaient subi peu de degfts.

24.75

22.00

to 19.25

U3 16.50

a 13.75

u) 11.00

c 8.25 3

5.50

2.75

Landings

Value

.........

720

640

560

480 12

400 0 -o

320 F =

240 ;11

160

80

INTRODUCTION

Annual soft-shell clam production in the Maritimes fluctuated around 9 million pounds during the period 1935-45. It rose to an all-time high of 23 million pounds in 1950. A dramatic fall in production then took place, reaching a low of 1.25 million pounds in 1963 (Fig. 1).

047

1 1 I I I I I I I I 0 51. 55 59 63 67 71 72

Years FIG . 1. Landings and value of the soft-shell clam fishery (Mya aernaria) in the Maritimes, 1947-1972.

This decline is associated with natural predation by clam drills (Lunatia heros), green crabs (Carcinides maenas), winter flounders (Pleuronectes americanus) and herring gulls (Larus argentatus) (Medcof and MacPhail 1964). Other factors which have contributed to the decreased landings include overfishing of clam stocks; failure to comply with size regulations; deple-tion of clam stocks from unstable sand flats, which failed to repopulate due to their shifting nature (smothering); and paralytic shellfish poisoning closures. Medcof and MacPhail (1964) demonstrated that one of the major causes for the decline could be associated with the harvesting tool-the clam hack. This method of harvesting was very destructive to unharvested clams and, as a result of repeated digging of the flats, contri-buted to the drastic decline.

Fishing effort decreased in the late 1950's and early 1960's, and allowed the beaches to rejuvenate. Since 1964, production has increased steadily, to 8.5 million pounds in 1971. However, the general condition of the clam flats indicates that a decline in production is imminent again. The detrimental effects of

2

conventional harvesting methods and an increasing problem of shellfish growing areas being closed because of domestic and industrial pollution appear to be major problems facing the fishery in the immediate future.

Various companies and individuals in the Maritimes have expressed interest in constructing depuration facilities to make use of the presently unutilized stocks in mildly contaminated areas. If a contaminated shellfish resource is to be utilized for a continuing depuration process, it should be managed in terms of maximum sustainable yield. To ensure this, harvesting methods must be evaluated and controlled.

Many researchers have demonstrated that the major control-lable factor regulating clam abundance in exploited areas is harvesting method. Needler and Ingalls (1944) have shown that mortality related to digging with clam hacks contributes more to population reduction than direct removal of marketable clams. Mechanical or hydraulic diggers have been used successfully to harvest shellfish for a number of years. These diggers have proven to be labour saving, more efficient and less destructive than the clam hack.

Mr. Cyril Price of Courtenay, British Columbia, developed a self-propelled, dry-flat clam digger, using the Bourne-type digger's principle of operation (Bourne 1967). This digger appeared to have some potential in the Maritimes, where the beaches frequently cover large areas and are relatively free of rock.

Pictures and drawings were obtained of the Price-Bourne digger, and modifications were applied to suit beach conditions in this area. Construction started in April, 1973, at the Biological Station at Ellerslie, Prince Edward Island; and, during the summer, experimental fishing trials were conducted in Musquodoboit Harbour, Nova Scotia.

DESCRIPTION AND OPERATION OF EQUIPMENT

The hydraulic unit for the digger consists of a Monarch 2-x 2-in. centrifugal pump, driven by a 4-hp Kohler, 4-cycle engine. The pump unit is floated in a skiff, and the water intake is a 2-in. "Kanaflex" suction hose, about 8 ft long. The suction hose is fitted with a 2-in. foot-valve and strainer at the free end, and standard 2-in. brass, "King" combination connectors to assemble the intake to the pump. The pump dis-charge is connected to the digger by 200 ft of 2-in., lay-flat vinyl hose in two 100-ft sections, joined together by brass, "King" connectors.

The digger is L-shaped in design (Fig. 2), with the manifold acting as the frame. The manifold is made of 2-in.-diameter pipe, with the front of the digger 3 ft 6 in. in width and the side 3 ft 4 in. in length. The digger is supported by three 8-

FIG. 2. Perspective drawing of experimental hydraulic clam digger.

in. pneumatic, wheelbarrow wheels joined to the manifold frame. The wheels are adjustable, so that the digger's height above the beach can be regulated.

For propulsion, the side of the digger supports a 4-hp motor, with a 1:6 reduction gear. This motor drives the digger through a pulley and winch system. The speed is manipulated by changing the size of the pulleys attached to the take-up winch. A hay-rake type pivot-levet is used as a clutch to engage and disengage the pulley system and control the digger's motion.

Ten nozzles are located on the front of the digger manifold. They are attached to the manifold by threaded, 3/8-in. to 1/2-in. reducing couplings. The nozzles are 3-in. lengths of 1/2-in. pipe, located along the manifold at 2-in. centers, and are flattened at the open end to create a spray effect. The nozzles are set either perpendicularly to the beach or at a 30®' angle towards the rear of the digger, with their height 1 in. or 2 in. above the beach. A gauge is mounted on the manifold to record water pressure at the manifold head. The normal working pressure is 25-30 lb/in.2, and the volume of water delivered, about 135 U.S. gpm.

Modifications were incorporated into the digger to adapt it to Maritime clam flats. These include: (1) a variable drive system to allow for different digging speeds, (2) pivot-lever engagement of the drive system, (3) adjustable wheels to manipul-ate the nozzle height, and (4) the use of lay-flat vinyl hose which is more economical and easier to handle in cold weather.

The total cost of the digger-including material, engines, pump and labour-was $850.00.

MacPhail and Medcof (1961) demonstrated that hydraulic

3

Miles

ATLANTIC OCEAN

4

digging is basically a flotation phenomenon. The water through the nozzles creates a fluid, soil-water emulsion, with a high specific gravity. In this heavy suspension, the low specific gravity clams rise to the surface like corks.

METHODS

The hydraulic digger's performance trials were conducted at Musquodoboit Harbour, along the eastern shore of Nova Scotia, during May and June of 1973. This area was chosen so that the digger's efficiency could be evaluated on clam beaches which were generally representative of the Maritimes. Two specific areas (Fig. 3) were selected for test sites: Coke Flat, a muddy-sand

FIG. 3. Map of Musquodoboit Harbour area, Nova Scotia, showing experimental clam digging sites at Coke Flat, Halifax County, 1973.

type of beach, located on the north side of Martinique Beach; and Baker's Island, a hard mud-sand type of beach, located on the north side of Francis Nose Island. The mean density of clams on Coke Flat was 5.7 per ft2 and on Baker's Island, 3.9 per ft2.

All performance trials were conducted at low tide, when the flats were dry. The plots were 19 in. (1.58 ft) in width and either 50 or 100 ft in length. The digging speed was varied, with ranges of 4-5, 9-10, 13-14, 16-17 and 19-20 ft/min. Digging

5

pressure was maintained at 25-30 lb/in.2 and water delivery at 135 U.S. gpm. The digger was operated parallel to the shore-line, back and forth in a flattened "S" pattern along the beach.

Experimental clam digger shown in operation.

Population estimates of each individual plot were determined by redigging and screening the plots by hand the day following the trial. From the total numbers found, the harvesting efficiency of the digger was calculated.

Other plots, 25 ft in length, were dug periodically, and the clams were left in the trench to enable observers to deter-mine the rate at which they would dig themselves back into the beach.

Observations on breakage, mortality, size and weight of clams, associated species, depth and condition of emulsion in trench, and general beach conditions were recorded the day following digging operations.

Length-frequency distribution was determined by measuring the clams harvested from two 50-ft plots dug at each work site.

6

RESULTS

Population Structure of Coke Flat

The length-frequency distribution of soft-shell clams on Coke Flat (Fig. 4) was determined by sampling the clams obtained

50.8 mm limit

15-

1 7 I ft I I I

0 10 20 30 40 Length (mm)

FIG. 4. Population structure of soft-shell clams sampled on Coke Flat, Halifax County, in 1973.

from the test plots (Table 1). The Coke Flat area was repre-sented by all year-classes, which were evenly distributed over the beach. However, 1971 and 1972 yeat-class clams were poorly represented. There was little evidence of harvesting activity in this area, since approximately one-half of the population was of commercial size (2 in. [50.8 mm] or greater). There was evidence of some growth stunting to the clam population in the uppet tidal levels of the flat, but average growth rate (2 in. in 5 yr) appeared similar to those of other populations in Musquodoboit Harbour.

Population Structure of Baker's Island

The length-frequency distribution of soft-shell clams on Baker's Island (Fig. 5) was determined by sampling the clams harvested from the test plots (Table 2). The Baker's Island population was somewhat more scattered within the test plots than that of Coke Flat. There was a predominance of sub-commercial size clams (<2 in. or 50.8 mm), with approximately

50 60 70

7

TABLE 1. Soft-shell clam harvest and hydraulic digging trials on Coke Flat, (Plot size: No. 1, 100-ft length and 50-ft length and 79-ft 2 area.)

extent of breakage during Halifax County, 1973.

158-ft2 area; all others,

Date Plot no.

Digging speed

(ft/min)

Nozzle adjustment Quantity harvested Extent of breakage. Height

(in.) Anglel

(°) Number Weight

(lb) No. %

May

9 1 13-14 1 90 1,048 47.5 7 0.7 9 2 13-14 1 90 475 23.8 0 0.0 9 3 13-14 1 90 458 23.0 1 0.2 9 4 13-14 1 30 363 14.8 0 0.0

10 5 13-14 1 30 489 24.3 0 0.0 10 6 13-14 2 30 271 11.5 0 0.0 10 7 13-14 2 30 265 11.4 0 0.0 10 8 13-14 2 90 451 22.8 1 0.2 11 9 13-14 2 90 339 14.8 0 0.0

11 10 19-20 1 30 285 13.2 0 0.0 11 11 19-20 1 30 298 15.4 0 0.0 11 12 19-20 1 90 223 10.9 0 0.0 14 13 19-20 1 90 218 9.6 1 0.5 14 14 19-20 2 90 208 9.9 0 0.0 14 15 19-20 2 90 196 9.8 3 1.5 14 16 19-20 2 30 264 14.9 0 0.0 15 17 19-20 2 30 280 15.2 0 0.0

15 18 4-5 1 90 381 15.6 8 2.1 15 19 4-5 1 90 474 23.5 12 2.5 15 20 4-5 1 30 569 29.1 5 0.9 16 21 4-5 1 30 389 15.2 0 0.0 16 22 4-5 2 30 284 13.5 3 1.1 16 232 4-5 2 30 498 24.1 4 0.8 16 242 4-5 2 90 378 15.5 6 1.6 17 25 4-5 2 90 474 23.6 1 0.2

17 26 9-10 1 30 405 16.1 0 0.0 17 27 9-10 1 30 324 14.8 0 0.0 17 28 9-10 1 30 461 23.1 1 0.2 18 29 9-10 1 30 438 21.9 0 0.0

18 30 16-17 1 30 365 15.2 0 0.0 18 31 16-17 1 30 284 13.1 0 0.0 18 32 16-17 1 30 298 15.1 0 0.0 18 33 16-17 1 30 334 15.3 0 0.0

1 90° = perpendicular to the beach. 30° = 30° off the perpendicular, away from the direction of travel.

2Plots used to determine population structure.

8

TABLE 2. Soft-shell clam harvest and extent of breakage during hydraulic digging trials on Baker's Island, Halifax County, 1973. (Plot size: All 50-ft length and 79-ft2 area.)

Date Plot no.

Digging speed

(ft/min)

Nozzle adjustment Quantity harvested Extent of breakage Height

(in.) Angle1

(°) Number Weight

(lb) No. %

May

28 1 4-5 1 90 245 9.8 11 4.5 28 2 4-5 1 90 238 9.6 9 3.8 28

32 4-5 1 30 246 9.6 4 1.6

28 4 2 4-5 1 30 268 10.4 7 2.6 29 5 4-5 2 30 239 9.5 4 1.7 29 6 4-5 2 30 244 9.7 7 2.9 29 7 4-5 2 90 214 8.0 6 2.8 29 8 4-5 2 90 199 7.5 2 1.0

30 9 13-14 1 30 35 1.4 0 0.0 30 10 13-14 1 30 42 1.6 0 0.0 30 11 13-14 1 90 30 1.3 1 3.3 30 12 13-14 1 90 32 1.3 3 9.4 31 13 13-14 2 90 24 1.0 6 25.0 31 14 13-14 2 90 28 1.1 1 3.6 31 15 13-14 2 30 30 1.2 0 0.0 31 16 13-14 2 30 32 1.2 2 6.3

June

1 17 19-20 1 30 14 0.5 0 0.0 1 18 19-20 1 30 18 0.6 0 0.0 1 19 19-20 1 90 16 0.5 1 6.3 1 20 19-20 1 90 16 0.5 0 0.0

11 21 19-20 2 90 15 0.5 0 0.0 11 22 19-20 2 90 17 0.6 0 0.0 11 23 19-20 2 30 14 0.5 0 0.0 11 24 19-20 2 30 15 0.5 0 0.0

12 25 9-10 1 30 32 1.3 1 0.8 12 26 9-10 1 30 38 1.5 1 0.7 12 27 9-10 1 30 30 1.2 3 2.3 12 28 9-10 1 30 39 1.6 0 0.0

13 29 16-17 1 30 21 0.9 1 4.8 13 30 16-17 1 30 25 1.1 0 0.0 13 31 16-17 1 30 21 0.8 0 0.0 13 32 16-17 1 30 23 1.0 0 0.0

190° = perpendicular to the beach. 30° = 30° off the perpendicular, away from the direction of travel.

2Plots used to determine population structure.

20-

o

0 10- C)

5-

25-

9

50.8 mm limit

0 10

Length (mm)

FIG. 5. Population structure of soft-shell clams sampled on Baker's Island, Halifax County, in 1973.

1/7 (14%) of the population being 50 mm or greater. This was probably due mainly to the fact that the area had been dug heavily during the previous two years. Here also, year-classes for 1971 and 1972 were poorly represented, and the growth rate on the flat (2 in. in 5 yr) appeared consistent with other populations throughout Musquodoboit Harbour.

Efficiency of Hydraulic Digger

The efficiency of the hydraulic clam digger was affected by several factors, such as the speed it travels over the beach, the compactness of the beach substrate, the nozzle angle and the nozzle height.

Effect of Digging Speed on Harvesting Efficiency

When the digger was operated at Coke Flat, a muddy-sand beach, the soil was easily washed away, and the harvesting efficiency was quite high at most speeds (Fig. 6). At speeds of 4-5, 9-10, and 13-14 ft/min, the efficiencies were 95.0%, 94.2% and 91.7% respectively. The digger's efficiency declined rapidly at higher speeds, with harvesting rates of 67.7% and 56.3% at 16-17 and 19-20 ft/min (calculated by number).

At Baker's Island (Fig. 6), where the soil was a hard mud-sand mixture, the digger's efficiency (83.3%) appeared acceptable only at an operating speed of 4-5 ft/min. The faster digging speeds of 9-10, 13-14, 16-17 and 19-20 ft/min resulted in harvesting efficiencies of 56.5%, 12.5%, 6.0% and 4.9% respectively (calculated by number).

20 30 40 50 60 70

Baker's Island

*100- Coke Flat 96-

I 64-B

e 48- o a.

32-

16-

* 100% represents population estimate

•■■••■■••■

10

4-5 9-10 13-14

16-17

19-20

DiggIngspeed(ft/mln) FIG. 6. Harvesting efficiency of experimental hydraulic clam digger during 1973 trials at Coke Fiat and Baker's Island, N.S.

The efficiency of the digger, when calculated by the weight of clams harvested, on Coke Flat was 6.6% lower than when calculated by number (Table 3). On Baker's Island this was not the case, for when the efficiency was calculated by weight it was 2.3% higher than when calculated by number (Table 4). These calculations are overall averages, based on both nozzle angles and all digging speeds tested.

This discrepancy in the efficiency rates could be attri-buted to the difference in the abundance of larger clams (2-in. length or greater) between the two test areas. The digger did not harvest some of the larger clams due to their depth in the substrate on Coke Flat, where on Baker's Island, the larger clams had already been harvested and therefore the remaining smaller clams were more accessible due to their shallower burrows in the substrate.

Effect of Nozzle Angle on Harvesting Efficiency

The nozzles of the digger were adjusted to two different positions in relation to the beach. The first setting was perpendicular to the ground, and the second was at a 30° angle

11

TABLE 3. Harvesting efficiency of hydraulic digger operated at different speeds on Coke Flat, Halifax County, 1973.

Nozzle adjustment Population estimate Clams harvested Percent efficiency Plot Height Angle Number Weight Number Weight By number By Weight no. (in.) (°) (lb) (1b)

Digging speed: 4-5 ft/min

18 1 90 401 17.4 381 15.6 95.0 89.7 19 1 90 494 26.5 474 23.5 96.0 88.7 20 1 30 593 31.3 569 29.1 96.0 93.0 21 1 30 401 14.7 389 15.2 97.0 103.4 22 2 30 299 16.2 284 13.5 95.0 83.3 23 2 30 530 27.6 498 24.1 94.0 87.3 24 2 90 407 17.7 378 15.5 92.9 87.6 25 2 90 504 25.1 474 23.6 94.1 94.0

Average 95.0 90.7


26 1 30 422 18.8 405 16.1 96.0 85.6 27 1 30 356 15.3 324 14.8 91.0 96.7 28 1 30 485 25.3 461 23.1 95.1 91.3 29 1 30 466 24.3 438 21.9 94.0 90.1

Average 94.2 90.7


1 1 90 1,103 56.0 1,048 47.5 95.0 84.8 2 1 90 511 24.3 475 23.8 93.0 97.9 3 1 90 498 24.9 458 23.0 92.0 92.4 4 1 30 382 17.6 363 14.8 95.0 84.1 5 1 30 520 26.9 489 24.3 94.0 90.3 6 2 30 305 13.9 271 11.5 88.9 82.7 7 2 30 301 26.8 265 11.4 88.0 67.9 8 2 90 524 27.5 451 22.8 86.1 82.9 9 2 90 390 17.3 339 14.8 86.9 85.6

Average 91.7 82.4


30 1 30 562 24.4 365 15.2 64.9 62.3 31 1 30 406 18.3 284 13.1 70.0 71.6 32 1 30 426 22.9 298 15.1 70.0 65.9 33 1 30 499 24.8 334 15.3 66.9 61.7

Average 67.7 53.9


10 1 30 445 22.6 285 13.2 64.1 58.4 11 1 30 481 24.8 298 15.4 62.0 62.1 12 1 90 406 19.9 223 10.9 54.9 54.8 13 1 90 411 20.3 218 9.6 53.0 47.3 14 2 90 408 21.4 208 9.9 51.0 46.3

15 2 90 400 20.3 196 9.8 49.0 48.3 16 2 30 455 24.7 264 14.9 58.0 60. 3 17 2 30 500 28.8 280 15.2 56.0 52.8

Average 56.3 54.1

12

TABLE 4. Harvesting efficiency of hydraulic digger operated at different speeds on Baker's Island, Halifax County, 1973.

Nozzle adjustment Population estimate Clams harvested Percent efficiency Plot Height Angle no. (in.) (°)

Number Weight (lb)

Number Weight By number (1h)

By weight


1 1 90 275 10.7 245 9.8 89.1 91.6 2 1 90 288 10.1 238 9.6 82.6 95.1 3 1 30 270 9.9 246 9.6 91.1 97.0 4 1 30 285 11.0 268 10.4 94.0 94.6 5 2 30 269 9.8 239 9.5 88.9 96.9 6 2 30 287 10.1 244 9.7 85.0 96.0 7 2 90 271 8.9 214 8.0 79.0 89.9 8 2 90 358 8.5 199 7.5 55-6 88.2

Average 83.2 93.7


25 1 30 210 9.5 132 5.3 62.9 55.8 26 1 30 238 9.2 138 5.5 58.0 59.8 27 1 30 236 9.0 130 5.2 55.1 57.8 28 1 30 267 8.9 139 5.6 52.1 62.9

Average 57.0 59.1


9 1 30 233 9.0 35 1.4 15.0 15.6 10 1 30 247 9.4 42 1.6 17.0 17.0 11 1 90 231 9.2 30 1.3 13.0 14.1 12 1 90 246 9.9 32 1.3 13.0 13.1 13 2 90 218 9.6 24 1.0 11.0 10.4 14 2 90 400 15.9 28 1.1 7.0 6.9 15 2 30 272 12.9 30 1.2 11.0 9.3 16 2 30 246 9.5 32 1.2 13.0 12.6

Average 12.5 12.4


29 1 30 263 10.3 21 0.9 8.0 8.7 30 1 30 357 14.5 25 1.1 7.0 7.6 31 1 30 525 23.9 21 0.8 4.0 3.4 32 1 30 460 23.2 23 1.0 5.0 4.3

Average 6.0 6.0


17 1 30 280 10.9 14 0.5 5.0 4.6 18 1 30 257 10.1 18 0.6 7.0 5.9 19 1 90 350 14.5 16 0.5 4.6 3.5 20 1 90 267 10.5 16 0.5 6.0 4.8 21 2 90 450 23.1 15 0.5 3.3 2.2 22 2 90 367 14.7 17 0.6 4.6 4.1 23 2 30 266 10.3 14 0.5 5.3 4.9 24 2 30 548 24.6 15 0.5 2.7 2.0

Average 4.8 4.0

13

off the perpendicular, away from the direction the digger was going. The different nozzle angles were tested at three speeds, 4-5, 13-14 and 19-20 ft/min, at both test locations.

On Coke Flat, the digger efficiency at the perpendicular setting was only 0.8% greater than that at the 30° setting, when calculated by number (Table 5). The difference was more signi-ficant, being 5.2% greater for the perpendicular setting, when calculated by weight. This could be attributed to the popula-tion structure of the beach.

When calculated by number, the efficiency of the digger on Baker's Island with the nozzle angle set at the 30° position was 5.6% greater than at the perpendicular position (Table 6). When calculated by weight, efficiency at the 30° setting was 2.7% greater than at the perpendicular position.

All above calculations are overall averages, based on the two different nozzle heights and all digging speeds tested.

The angle of the nozzles appears to affect the digging efficiency significantly, especially on firmer substrates such as at Baker's Island. When the nozzle angle is set at 30° off the perpendicular towards the rear of the digger, the harvesting ability is increased. The 30° setting provides a more efficient penetration and washing action of the substrate, therefore releasing more clams from their burrows and enabling them to float to the surface of the trench. There were also the findings of Wallace' on his work with the Maryland harvester when digging in firm substrates.

Effect of Nozzle Height on Harvesting Efficiency

The effect of nozzle height above the beach was evaluated by adjusting the manifold. The nozzle height was determined by measuring the perpendicular distance from the end of the nozzle to the beach. Heights of one and two inches were tested, at each site, at digging speeds of 4-5, 13-14 and 19-20 ft/min, and at both nozzle angle settings. The following calculations are over-all averages, based on the combined results of the two nozzle angles and three digging speeds tested.

On Coke Flat, harvesting efficiency at the 1-in. nozzle height was 7.5% greater when calculated by number and 11.3% greater when calculated by weight than at the 2-in. nozzle height (Table 7).

On Baker's Island, the harvesting efficiency at the 1-in. nozzle height was 6.0% greater when calculated by number and 2.8% greater when calculated by weight than at the 2-in. nozzle height (Table 8).

'Personal communication, 1973, from Wallace, D.A., Assistant Director of Marine Research, Dept. of Marine Resources, State of Maine, U.S.A.

14

TABLE 5. Harvesting efficiency of hydraulic digger operating at different nozzle angle adjustments on Coke Flat, Halifax County, 1973.

Population estimate Clams harvested Percent efficiency Plot Digging speed Nozzle height Number Weight Number Weight By number By weight no. (ft/min) (in.) (11)) (l13)

Nozzle angle perpendicular to ground

18 4-5 1 401 17.4 381 15.6 95.0 89.7 19 4-5 1 494 26.5 474 23.5 96.0 88.7 24 4-5 2 407 17.7 378 15.5 92.9 87.6 25 4-5 2 504 25.1 474 23.6 94.1 94.0

1 13-14 1 1,103 56.0 1,048 47.5 95.0 84.8 2 13-14 1 511 24.3 475 23.8 93.0 97.9 3 13-14 1 498 24.9 458 23.0 92.0 92.4 8 13-14 2 524 27.5 451 22.8 86.1 82.9 9 13-14 2 390 17.3 339 14.8 86.9 85.6

12 19-20 1 406 19.9 223 10.9 54.9 54.8 13 19-20 1 411 20.3 218 9.6 53.0 47.3 14 19-20 2 408 21.4 208 9.9 51.0 46.3 15 19-20 2 400 20.3 196 9.8 49.0 48.3

Average 82.4 78.6

Nozzle angle 30° from perpendicular toward rear of digger

20 4-5 1 593 31.3 569 29.1 96.0 93.0 21 4-5 1 401 14.7 389 15.2 97.0 103.4 22 4-5 2 299 16.2 284 13.5 95.0 83.3 23 4-5 2 530 27.6 498 24.1 94.0 87.3

4 13-14 1 382 17.6 363 14.8 95.0 84.1 5 13-14 1 520 26.9 489 24.3 94.0 90.3 6 13-14 2 305 13.9 271 11.5 88.9 82.7 7 13-14 2 301 26.8 265 11.4 88.0 67.9

10 19-20 1 445 22.6 285 13.2 64.1 58.4 11 19-20 1 481 24.8 298 15.4 62.0 62.1 16 19-20 2 455 24.7 264 14.9 58.0 60.3 17 19-20 2 500 28.8 280 15.2 56.0 52.8

Average 81.6 73.4

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TABLE 6. Harvesting efficiency of hydraulic digger operated at different nozzle angle adjustments on Baker's Island, Halifax County, 1973.

Population estimate Clams harvested Percent efficiency Plot Digging speed no. (ft/min)

Nozzle height (in.)

Number Weight (lb)

Number Weight By number By weight

Nozzle angle perpendicular to ground

1 4-5 1 275 10.7 245 9.8 89.1 91.6 2 4-5 1 288 10.1 238 9.6 82.6 95.1 7 4-5 2 271 8.9 214 8.0 79.0 89.9 8 4-5 2 358 8.5 199 7.5 55.6 88.2

11 13-14 1 231 9.2 30 1.3 13.0 14.1 12 13-14 1 246 9.9 32 1.3 13.0 13.1 13 13-14 2 218 9.6 24 1.0 11.0 10.4 14 13-14 2 400 15.9 28 1.1 7.0 6.9

19 19-20 1 350 14.5 16 0.5 4.6 3.5 20 19-20 1 267 10.5 16 0.5 6.0 4.8 21 19-20 2 450 23.1 15 0.5 3.3 2.2 22 19-20 2 367 14.7 17 0.6 4.6 4.1

Average 30.7 35.3

Nozzle angle 30° from perpendicular toward rear of digger

3 4-5 1 270 9.9 246 9.6 91.1 97.0 4 4-5 1 285 11.0 268 10.4 94.0 94.6 5 4-5 2 269 9.8 239 9.5 88.9 96.9 6 4-5 2 287 10.1 244 9.7 85.0 96.0

9 13-14 1 233 9.0 35 1.4 15.0 15.6 10 13-14 1 247 9.4 42 1.6 17.0 17.0 15 13-14 2 272 12.9 30 1.2 11.0 9.3 16 13-14 2 246 9.5 32 1.2 13.0 12.6

17 19-20 1 280 10.9 14 0.5 5.0 4.6 18 19-20 1 257 10.1 18 0.6 7.0 5.9 23 19-20 2 266 10.3 14 0.5 5.3 4.9 24 19-20 2 548 24.6 15 0.5 2.7 2.0

Average 36.3 38.0

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TABLE 7. Harvesting efficiency of hydraulic digger operated at different nozzle heights on Coke Flat, Halifax County, 1973.

Population estimate Clams harvested Percent efficiency Plot no.

Nozzle

Digging speed (ft/min)

Nozzle adjust. angle (°)

Number Weight (lb)

Number Weight. By number (lb)

By weight

height adjusted 1 inch above ground

18 4-5 90 401 17.4 381 15.6 95.0 89.7 19 4-5 90 494 26.5 474 23.5 96.0 88.7 20 4-5 30 593 31.3 569 29.1 96.0 93.0 21 4-5 30 401 14.7 389 15.2 97.0 103.4

1 13-14 90 1,103 56.0 1,048 47.5 95.0 84.8 2 13-14 90 511 24.3 475 23.8 93.0 97.9 3 13-14 90 498 24.9 458 23.0 92.0 92.4 4 13-14 30 382 17.6 363 14.8 95.0 84.1 5 13-14 30 520 26.9 489 24.3 94.0 90.3

10 19-20 30 445 22.6 285 13.2 64.1 58.4 11 19-20 30 481 24.8 298 15.4 62.0 62.1 12 19-20 90 406 19.9 223 10.9 54.9 54.8 13 19-20 90 411 20.3 218 9.6 53.0 47.3

Average 85.3 81.3

Nozzle height adjusted 2 inches above ground

22 4-5 30 299 16.2 284 13.5 95.0 83.3 23 4-5 30 530 27.6 498 24.1 94.0 87.3 24 4-5 90 407 17.7 378 15.5 92.9 87.6 25 4-5 90 504 25.1 474 23.6 94.1 94.0

6 13-14 30 305 13.9 271 11.5 88.9 82.7 7 13-14 30 301 26.8 265 11.4 88.0 67.9 8 13-14 90 524 27.5 451 22.8 86.1 82.9 9 13-14 90 390 17.3 339 14.8 86.9 85.6

14 19-20 90 408 21.4 208 9.9 51.0 46.3 15 19-20 90 400 20.3 196 9.8 49.0 48.3 16 19-20 30 455 24.7 264 14.9 58.0 60.3 17 19-20 30 500 28.8 280 15.2 56.0 52.8

Average 77.8 70.0

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TABLE 8. Harvesting efficiency of hydraulic digger operated at different nozzle heights on Baker's Island, Halifax County, 1973.

Population estimate Clams harvested Percent efficiency Plot no.

Digging speed (ft/min)

Nozzle adjust. angle (°)

Number Weight (lb)

Number Weight By number (lb)

By weight

Nozzle height adjusted 1 inch above ground

1 4-5 90 275 10.7 245 9.8 89.1 91.6 2 4-5 90 288 10.1 238 9.6 82.6 95.1 3 4-5 30 270 9.9 246 9.6 91.1 97.0 4 4-5 30 285 11.0 268 10.4 94.0 94.6

9 13-14 30 233 9.0 35 1.4 15.0 15.6 10 13-14 30 247 9.4 42 1.6 17.0 17.0 11 13-14 90 231 9.2 30 1.3 13.0 14.1 12 13-14 90 246 9.9 32 1.3 13.0 13.1

17 19-20 30 280 10.9 14 0.5 5.0 4.6 18 19-20 30 257 10.1 18 0.6 7.0 5.9 19 19-20 90 350 14.5 16 0.5 4.6 3.5 20 19-20 90 267 10.5 16 0.5 6.0 4.8

Average 36.5 38.1

Nozzle height adjusted 2 inches above ground

5 4-5 30 269 9.8 239 9.5 88.9 96.9 6 4-5 30 287 10.1 244 9.7 85.0 96.0 7 4-5 90 271 8.9 214 8.0 79.0 89.9 8 4-5 90 358 8.5 199 7.5 55.6 88.2

13 13-14 90 218 9.6 24 1.0 11.0 10.4 14 13-14 90 400 15.9 28 1.1 7.0 6.9 15 13-14 30 272 12.9 30 1.2 11.0 9.3 16 13-14 30 246 9.5 32 1.2 13.0 12.6

21 19-20 90 450 23.1 15 0.5 3.3 2.2 22 19-20 90 367 14.7 17 0.6 4.6 4.1 23 19-20 30 266 10.3 14 0.5 5.3 4.9 24 19-20 30 548 24.6 15 0.5 2.7 2.0

Average 30.5 35.3

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The results from each test location are similar when the harvesting efficiency is calculated by number (7.5% and 6.0% difference). This would indicate that the 1-in. height was more efficient than the 2-in. height because of increased penetration by the water jets. When calculated by weight, large difference in digging efficiency between the 1-in. and 2-in. nozzle heights at the two test locations, 11.3% and 2.8% respectively, could be attributed to the different population structures, as there was a larger number of small clams on Baker's Island than on Coke Flat. It is also possible that during the test runs a heavy concentra-tion of small clams was encountered, thereby artificially decreasing the efficiency of the 1-in. nozzle height below that of the 2-in. nozzle height due to the smaller clams being nearer to the surface and more available.

Breakage and Condition of Clams

The shellstock harvested by the hydraulic digger had little breakage, ranging from 0.0% to 2.5%, with an average of 0.4% for 33 plots dug on Coke Flat (Table 1). The breakage for Baker's Island was somewhat higher, ranging from 0.0% to 25.0%, with an average of 2.6% for 32 plots dug (Table 2). However, breakage was generally between 0.0% and 4.8% (average of 1.3%) at this site, and the high breakage shown for plots 12, 13, 16 and 19 can probably be attributed to poor handling of the sampled shell-stocks rather than the harvesting procedure.

Problems Associated with Improper Use of the Digger

The digger had two noticeable drawbacks when operated improperly. MacPhail and Medcof (1961) noted damage to the beach by "welling", which results in a large hole being dug in the beach when the digger is left stationary with the pump engine running. These wells fill in slowly and act as traps for the small clams. This could cause significant mortality if widely practised. In the present study, a similar problem was observed. It was found that the hole, after filling, still remained very soft and presented a hazard to those walking on the beach, even after being covered at high tide.

The other problem occurred when the digger was operating on a sloped beach. There was a tendency of the smaller clams (1-in. length or less) to wash out of the trench, making them suscept-ible to predation by gulls. If they remained in the trench they were able to reestablish themselves very quickly (15-20 min) and were less likely to be lost to the fishery.

It was found that on clam flats of a sandy nature, such as Coke Flat, the most efficient speed in terms of production/hour (293.5 lb) was 16-17 ft/min (Table 9). However, harvesting efficiency at this speed drops to 68% (Fig. 6). At 13-14 ft/min, production per hour was 266.2 lb, with a harvesting efficiency of 90.7%. It would therefore appear that a suitable speed for maintaining a high production and harvest efficiency is around 14-15 ft/min.

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Clams exposed in trench made by the hydraulic clam digger.

The population on Baker's Island is considered to be only of recreational value at best, and therefore the digger's overall performance could not be evaluated properly. With a pump capacity of 135 gpm, it would not be productive to utilize the digger on a beach with this type of substrate (mud) unless the population density was high enough to operate at 4-5 ft/min and maintain an adequate production (approx. 250 lb/hr).

The clams from Coke Flat appeared to have more fragile shells than those from Baker's Island and, therefore, no suitable explanation can be offered for the slightly higher rate of breakage on Baker's Island than on Coke Flat.

The outward appearance of the shellstock harvested with the mechanical digger was very clean, but there was an increase in the amount of sand and silt in the mantle cavity as compared to those harvested by conventional methods. This, however, would not present a problem after the clam meats have gone through a washing or depuration process.

General Observations

The hydraulic clam digger proved to be a very efficient and less labour-intensive method of harvesting than the hand hack. This is due to the fact that it is self-propelled, allow-ing the operator to collect the clams simultaneously as digging

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TABLE 9. Comparison of hydraulic clam harvester production at different digging speeds, on Coke Flat and Baker's Island, Halifax County, 1973.

Coke Flat (all plots) Baker's Island (all plots)

Digging speed (ft/min) 4-5 9-10 13-141 16-17 19-20 4-5 9-10 13-14 16-17 19-20

Diggirp speed (yd /hr) 51.8 99.3 127.6 175.5 196.8 46.1 86.0 139.5 146.8 195.9

Total digging time (min) 81.3 21.2 33.0 12.0 21.4 91.4 21.0 30.2 12.3 21.5

Area dug (yd') 70.2 35.1 70.2 35.1 70.2 70.2 35.1 70.2 35.1 70.2

Number of clams harvested/hr 2,544 4,608 5,656 6,405 5,529 1,243 397 503 439 348

Weight of clams harvested (lb/hr) 118.2 214.8 266.2 293.5 277.3 48.6 16.0 20.1 18.5 11.7

Number of clams harvested/yd 49.1 46.4 44.3 36.5 28.1 26.9 4.6 3.6 2.9 1.8

Weight of clams harvested (lb/yd2) 2.3 2.2 2.1 1.7 1.4 1.1 0.2 0.6 0.1 0.1

Number of clams harvested 3,447 1,628 3,111 1,281 1,972 1,893 339 253 90 125

Weight of clams harvested (lb) 160.1 75.9 146.4 58.7 98.9 74.1 5.6 10.1 3.8 4.2

Breakage of clams harvested (no.) 39 1 2 0 4 50 5 13 1 1

Breakage of clams harvested (%) 1.2 0.1 0.1 0.0 0.3 2.6 1.0 6.0 1.2 0.8

Does not include production from plot #1.

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progresses. Using conventional methods, considerable time is lost in the digging operation, and in switching continually from digging to collecting.

An important advantage of the hydraulic digger from a con-servation aspect is the reduced mortality of small clams left behind (<5%) (MacPhail and Medcof 1961). Compared to the clam hack, the hydraulic digger demonstrates its benefit as a management tool, as the hack will destroy up to 50% of the young clams left (Needler and Ingalls 1944).

The hydraulic digger, if adjusted properly and operated according to the beach substrate, harvests almost all the market-size clams in its path and, therefore, the area is not re-dug until the remaining shellfish have grown to market size. The clam hack fishermen, however, recover only about 60% of the market-size clams. The remaining resource is attractive enough that fishermen will re-dig the same ground several times a year, further damaging undersized stock and not allowing the beach to rejuvenate.

POTENTIAL IMPROVEMENTS.TO DIGGER

Improvements to the hydraulic digger may come from gaining experience during operation on different beach substrates. Such things as manifold size, nozzles, methods of propulsion and different types of running gear (skids instead of wheels) could be improved upon. The trials on Baker's Island suggest that a larger pump (200-250 gpm) is required to ensure effectiveness on the wide variety of beach substrates in this area.

SUMMARY

1. During May and June, 1973, hydraulic clam-harvesting trials were conducted on Coke Flat and Baker's Island in Musquodoboit Harbour, to determine the efficiency and stock-management impact of this method.

2. The digging efficiency increased as the speed decreased.

3. On Coke Flat, production (pounds harvested) was highest at 16-17 ft/min, and digging efficiency (percent of clams harvested) was highest at 4-5 ft/min.

4. Breakage of clam stocks averaged less than 3%.

5. Production of the digger was 2- to 5-fold higher than that of manual digging with the clam hack (Medcof 1961).

6. Efficiency and production were higher on sandflats than on mud flats.

7. Efficiency was higher when the nozzle height was 1 in. above the substrate as compared to 2 in.

8. Efficiency was greater with the nozzle set at 30° toward the rear of the digger than when set perpendicularly to the beach. The percentage increase in efficiency was dependent on substrate and speed.

9. Production and efficiency of the digger could be improved on firmer substrates, if water pressure and volume were increased.

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ACKNOWLEDGEMENTS

The authors wish to thank Mr. Cyril Price of Courtenay, British Columbia, who allowed us to experiment with his design of the hydraulic digger and modify it to suit the clam flats of the Maritimes.

Special thanks are extended to Mr. Hubert Hutchinson and Mr. Milton MacAusland of the Ellerslie Station, Prince Edward Island, who constructed the hydraulic digger; Mr. Bryce Kent, who prepared the plans; and Mr. Norwood Whynot, who prepared the illustrations.

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REFERENCES

Bourne, N. 1967. Digging efficiency trials with a hydraulic clam rake. Fisheries Research Board Technical Report, No. 15. 32 p.

MacPhail, J.S. 1961. Building and testing a hydraulic clam rake. Fish. Res. Bd. Canada, MS Report (Biol), No. 711. 10 p.

MacPhail, J.S. and J.C. Medcof. 1961. Fishing efficiency trials with a hydraulic clam rake. Fish. Res. Bd. Canada, MS Report (Biol), No. 724. 16 p.

Medcof, J.C. 1950. Burrowing habits and movements of soft-shell clams. Fish. Res. Bd. Canada, Atlantic Prog. Rept. No. 50: 17-22.

Medcof, J.C. 1961. Effect of hydraulic escalator harvestor on undersized soft-shell clams. 1959 Proc. National Shellfisheries Association, No. 50:151-161.

Medcof, J.C. and J.S. MacPhail. 1952. Breakage — the bugbear in clam handling. Fish. Res. Bd. Canada, Atlantic Prog. Rept. No. 54:19-25.

Medcof, J.C. and J.S. MacPhail. 1964. A new hydraulic rake for soft-shell clams. National Shellfisheries Association, Vol. 53, 1964. 31 p.

Needler, A.W.H. and R.A. Ingalls. 1944. Experiments in the production of soft-shell clams (Mya). Fish. Res. Bd. Canada, Atlantic Prog. Rept. No. 35:3-8.

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