Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 100-104. 1989

Tidal stream transport of larval fishes into non-stratified estuaries

Scott A. Holt, G. Joan Holt, and Connie R. Arnold

Holt, Scott A ., Holt, G. Joan, and Arnold, Connie R. 1989. Tidal stream transport of larval fishes into non-stratified estuaries. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 100-104.

Larvae and juveniles of several fish species utilize tidal flow as a mechanism for migration. Reliance on the differences in direction of the net non-tidal flow between surface and bottom appears to be a major strategy for transport and retention of fish larvae in partially-stratified estuaries. Vertical movement by fish up into the current stream, when the flow is in the “desired” direction of travel, and movement down to the bottom, out of the current stream, when the flow is in the opposite direction (a process termed “selective tidal stream transport”) facilitates the migration of juveniles and adults of several species. Tidal stream transport has been demonstrated in the larvae of only a few species and the generality of this process as the mechanism for larval transport into estuaries has not been established. Towed ichthyoplankton samples were taken on flood tide and the subsequent ebb tide at surface and bottom at five stations on a transect across the Aransas Pass tidal inlet, Texas to test the hypothesis that larval red drum (Sciaenops ocellatus) moved to the edges and/or moved to the bottom on ebb tide and, in both cases, moved into the water column on flood tide to take advantage of the reduced currents at the boundaries. There was no evidence of horizontal movement but larval red drum did appear to move vertically in response to tidal direction. Larval red drum were more abundant on the bottom than on the surface on both flood and ebb but the difference was much greater on the ebb flow. The response to tidal direction was weak compared to dramatic differences seen in some other species. There was little difference in density of larvae between flood and ebb tide but the mean size was larger on flood than ebb, suggesting that only the larger individuals are leaving the tidal plume and being retained in the estuary.

Scott A . Holt, G. Joan Holt, and Connie R. Arnold: The University o f Texas Marine Science Institute, P.O. Box 1267, Port Aransas, TX 78373, USA.

Introduction

Juveniles and adults of several fish species utilize tidal flow as a mechanism for migration through a process termed “selective tidal stream transport” (Greer Walker et al., 1978; Arnold and Cook, 1984). Selective tidal stream transport involves vertical movement by a fish up into the current stream, when the flow is in the “desired” direction of travel, and movement down to the bottom, out of the current stream, when the flow is in the opposite direction. Utilization of tidal stream transport has been documented in glass eels (Creutz- berg, 1961; McCleave and Kleckner, 1982) and plaice larvae (Rijnsdorp et a l., 1985), but not in weakly-swim- ming perciform larvae.

Mechanisms of larval fish transport in partially-stratified estuaries have been examined in some detail. Through vertical migration keyed to tidal stage, Atlantic croaker, spot, and southern flounder larvae could arrive

at and maintain a preferred position in the estuary in Cape Fear River, North Carolina (Weinstein et al.,1980). A similar response was shown for herring larvae in the Sheepscot estuary in Maine (Graham, 1972).

There have been few larval transport studies from non-stratified estuaries. There was no difference in the densities of flood and ebb tide catches of several species of fish larvae at the entrance to Whangateau Harbour, New Zealand (Roper, 1986). Differences in length- frequency distributions between tide stages for two species were attributed to the larger individuals moving in on flood tide and settling to the bottom so they did not wash back out on ebb tide. Postlarval plaice apparently utilize this mechanism to enter the Wadden Sea (Creutzberg et al., 1978; van der Veer, 1986). The generality of this process as the mechanism for larval transport into estuaries has not been established and other mechanisms have been proposed for some species

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(Tanaka, 1985). Postma (1961) showed that purely abiotic factors (in tidal inequalities) could transport material landward from tidal inlets.

Red drum larvae have been taken in several larval fish studies but experimental designs were inadequate to examine transport mechanisms. Red drum larvae were found to be most abundant in the surface waters compared to mid-depth or bottom on flood tides in Cedar Bayou, Texas (King, 1971). Conversely, Alls- house (1983) found no difference between surface and bottom densities of red drum larvae (nor for most other fish species) on flood tide in Aransas Pass, Texas. Neither study sampled ichthyoplankton on ebb tide.

Red drum spawn from late August to early October in the western Gulf of Mexico. The location of spawning is poorly known; however, it has been commonly assumed, based on the occurrence of small (8-15 mm) demersal juveniles near the inlets (Pearson, 1929; Holt and Arnold, 1982; Holt et al., 1983), that they spawn in coastal areas near the tidal passes. More recently, the collection of red drum eggs in the immediate vicinity of the Aransas Pass inlet (S. Holt, unpublished data) has proven that some portion of the red drum population spawns near the inlet. The presence of newly spawned (2 to 4 h old) eggs in night-time flood tides through the inlet and their absence during night-time ebb tides suggest that spawning occurs only outside the pass and not in the estuary (Holt et al., 1985).

In previous work (unpublished data), we examined the immigration pattern of red drum larvae through the tidal inlet into their estuarine nursery grounds. We measured the abundance of red drum eggs, planktonic larvae, and demersal post-larvae during flood tide along a transect leading from the inlet up into Aransas Bay. Red drum eggs and larvae were relatively abundant in surface plankton samples in “offshore” water which moved into the bay with flood tide but were very rare in “bay” water which had not mixed with “offshore” water from the flood tide. Conversely, demersal larvae (generally greater than 5 mm standard length) were found in seagrass meadows all along the transect. There was no increase in mean length with increasing distance from the inlet in the demersal fish, suggesting that these larvae moved either actively or passively up into the estuary well beyond the reach of flood tide before moving into their seagrass nursery areas. These results suggested that some behavioral activity of the larvae, such as vertical or horizontal movement in response to tidal flow, was responsible for getting them up into the estuary. None of the previously mentioned studies, including our own, has sampled both flood and ebb tides with synoptic sampling from shoreline to mid-channel.

In this paper the hypothesis is tested that some form of behavioral response to tidal cycles allows red drum to migrate rapidly through the tidal inlet from their offshore spawning sites into the estuarine nursery area. We proposed that this behavioral response would

involve movement to either the bottom or the sides of the tidal inlet on ebb tide and back into mid-channel on flood tide.

Methods

The sampling scheme was designed to determine the horizontal and vertical distribution of red drum larvae on flood and ebb tides. The study site consists of a transect established across Lydia Ann Channel (one of three distributaries of the Aransas Pass tidal inlet) at a point two kilometers from its intersection into Aransas Bay, Texas (Fig. 1). At this point the channel is approximately 400 m wide. Five stations were established along the transect. One station was situated in the middle of the channel in approximately 6 m of water and two stations, one on either side of the midline, were set near shore in approximately 3 m of water. These three stations will be referred to as the deep stations. The final two stations (shallow stations) were on each shoreline in

T E X A S

Aransas Bay

PortAransas

Gulf of M ex ico

\

Figure 1. Location of sampling area (marked with a solid circle) in the Aransas Pass tidal inlet, Texas, USA.

Salinometer. Currents were measured near the surface and near the bottom at each shoreline station and at the mid-channel station with an EN D EC O type 110 current meter. Currents were measured prior to, mid-way through, and following the collection of each suite of samples. Sampling was initiated 2 h prior to peak current flow for each tide and was completed within 3 h. Little variation in current speed was observed during any of the sampling periods. Tides along the Texas coast are mixed but principally diurnal with typical tropic tide currents of 50-60 cm s_1 and equatorial tide currents of 10-30 cm s-1 (Smith, 1979). Daily water level changes are on the order of 10-80 cm.

Analysis of density data from the shoreline sites was done separately due to the use of a different net size and the single-depth samples at those sites. The Wil- coxon rank-sum test was used to examine differences in density between flood and ebb tide for these shoreline sites. Elimination of the single depth shoreline sites from the analysis produced a factorial design of three sites and two depths with equal sample size. These data, however, did not meet the assumptions of analysis of variance even after transformation. A chi-square test for heterogeneity (McCleave et al. , 1987) was employed to test the null hypothesis that tide had no effect on depth distribution.

Length-frequency data did conform to the assumptions of analysis of variance (Kirk, 1982), i.e. the null

hypothesis that the data were a random sample from a normal distribution was not rejected using the Shapiro- Wilk statistic, W. A two-way analysis of variance was performed on mean length in each replicate sample with tide and depth as main effects.

Results

Five hundred and sixty-six larval red drum were taken in 192 samples during the study. Only 26 of these larvae were taken at the two shoreline sites. Densities at these sites were not significantly different between tides (p > 0.22). One entire suite of flood-ebb samples was eliminated from this and subsequent analysis because extremely high densities of ctenophores during the ebb tide resulted in zero catches of fish larvae of all species. The comparison of relatively high catches on flood with zero catches on ebb would have seriously skewed the results. The possibility that the zero catches at high ctenophore densities occur due to predation by the ctenophores (van der Veer, 1985) is of interest but beyond the scope of this study.

The chi-square test for heterogeneity showed a significant difference in depth distribution between flood and ebb tide (p < 0.025). Density was greater on the bottom than on the surface on both tides but the difference between surface and bottom densities was

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E F

Tide 1 5.4934 5.61 0.0202Depth 1 0.0117 0.01 0.9130Tide-Depth 1 0.1471 0.15 0.6992

1 Ebb

>“oz:LUZ>oLUCZLu 1

Flood

LENGTH CLASS (m m )

Figure 3. Length-frequency distributions for red drum larvae on flood and ebb tide for all sample dates at the deep stations.

bottom than on the surface on both ebb and flood. The tidal stream transport hypothesis predicts that larval density should be highest near the bottom on the ebb tide and higher away from the bottom, or at least become uniform throughout the water column, on the flood, for larvae to take advantage of favorable currents. The significant difference in depth distribution between flood and ebb tides shown by the heterogeneity chi- square test indicates some vertical movement into the water column on flood, and movement towards the bottom on ebb tide. Our sampling strategy, taking only surface and bottom samples, did not provide an indication of density at mid-levels in the water column and densities may have increased substantially there on the flood tide.

The weak swimming ability of even relatively large (4-6 mm) red drum larvae (G. J. Holt, unpublished observation) would intuitively suggest that they would not laterally traverse the 150-200 m from mid-channel to shoreline to take advantage of the weaker currents

4 .0 -

Flood Ebb

TIDE

Figure 4. Mean length and standard error of red drum larvae in flood and ebb tide samples at each depth. Catches are averaged over all sampling dates at the three deep sites.

KX>4 Bo ttom

K\N Surface

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there. Such behavior has been suggested for red drum (King, 1971) but there was no evidence of lateral movement to utilize the lower current speed along the shore in this study. A similar lack of lateral movement was observed for Gilchristella aestuarius in the Sundays River tidal inlet, South Africa (Melville-Smith et al.,1981).

One obvious aspect of the immigration process for red drum larvae is the size difference between flood and ebb. Creutzberg e/ al. (1978) postulated that older plaice larvae were stimulated by food resources to remain on the bottom on ebb tide, thereby avoiding being flushed back out of the estuary, while younger stage larvae were flushed from the estuary on ebb tide. Similar observations were made for another flatfish, Rhom- bosoleaplebeia, in Whangateau Harbour, New Zealand (Roper, 1986). The greater proportion of larger red drum on flood than ebb suggests that larger larvae are more effectively retained in the estuary (beyond the tidal inlet study area) than are small larvae. Tidal currents are substantially reduced in the shallower waters of the estuary than in the tidal inlet and older larvae are apparently more competent at getting out of the ebb tide current which would flush them back out of the estuary. O ur data do not provide an indication of where or how this differential retention takes place, but this is a subject of current research.

Acknowledgements

This research was funded by a program development grant from the Texas A&M University Sea Grant program. We extend our appreciation to L. Young-Abel, M. Denison, and T. Olsen for assistance in collecting and processing the samples.

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