larval fishes of a middle atlantic bight estuary: assemblage structure and temporal stability
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Larval fishes of a Middle Atlantic Bight estuary:assemblage structure and temporal stability
David A. Witting, Kenneth W. Able, and Michael P. Fahay
Abstract: We collected weekly, quantitative ichthyoplankton samples over 6 years (19891994, 1309 samples) toidentify temporal scales of variability in the abundance and occurrence of larval fish assemblages near Little Egg Inletin southern New Jersey, U.S.A. We collected species that spawn in the estuary (30%), both the estuary and continentalshelf (35%), continental shelf (25%), and the Sargasso Sea (10%). The following analyses suggest an annually repeatedseasonal progression of species assemblages: (i) the rank abundance of the 20 dominant species did not changesignificantly from year to year, (ii ) variation in the density of the dominant species was primarily explained byintraannual rather than interannual variation, and (iii ) multivariate analysis of the assemblage matrix identified fiveseasonal assemblages that occurred during all six years. We found that the timing and duration of each of theseseasonal groups were correlated with two characteristics of the annual temperature cycle, magnitude (higher or lowertemperature) and trajectory (increasing vs decreasing temperature). We suggest that the repeated occurrence of larvalfish assemblages in temperate estuaries along the U.S. coast may, in part, be driven by local environmental processes.
Rsum: Nous avons prlev hebdomadairement des chantillons quantitatifs dichtyoplancton durant 6 ans(19891994, 1 309 chantillons) pour tablir les chelles temporelles de variabilit de labondance et de la prsence degroupements de larves de poissons prs du bras Little Egg dans le sud du New Jersey, aux tats-Unis. Nous avonsrecueilli des espces qui frayent dans lestuaire (30%), la fois dans lestuaire et sur la plate-forme continentale(35%), sur la plate-forme continentale seulement (25%) et dans la mer des Sargasses (10%). Les rsultats suivantslaissent penser quil y a une progression saisonnire rpte annuellement des groupements despces : (i) lesabondances relatives des 20 espces dominantes nont pas chang significativement danne en anne, (ii ) la variationde la densit de lespce dominante sexpliquait principalement par une variation intra-annuelle plutt quinter-annuelleet (iii ) lanalyse multivarie de la matrice des groupements a permis de reprer cinq groupements saisonniers quitaient prsents durant les six annes. Nous avons observ que le moment de formation et la dure de chacun de cesgroupements saisonniers taient corrls avec deux caractristiques du cycle annuel de la temprature, soit le degr detemprature (temprature basse ou leve) et la tendance de la temprature (temprature croissante ou dcroissante).Nous pensons que la survenue rpte des groupements de larves de poissons dans les estuaires temprs de la cteamricaine peut en partie tre dtermine par des processus environnementaux locaux.
[Traduit par la Rdaction] Witting et al. 230
The orderly seasonal succession in abundance and speciescomposition of the dominant components of temperate estu-arine assemblages figures prominently in all general reviews(Allen and Barker 1990; McGovern and Wenner 1990). Geo-graphical variation in sources of larvae is also a commoncharacteristic of many estuarine fish assemblages (Frank andLeggett 1983; Suthers and Frank 1991; Hettler and Barker1992). There are, however, few published examples thatdemonstrate the repeatability of species assemblages andabundances from year to year. A large part of these seasonal
changes have been attributed to the transient nature of thedominant species such that the estuary is only used duringspecific life history stages (McGovern and Wenner 1990).Others have suggested that spawning is timed such that thearrival of early life history stages into the nursery groundstakes advantage of favorable conditions relative to feeding,predation, etc. (e.g., Frank and Leggett 1983), thus produc-ing a seasonal pattern in the species composition of larvaeentering the estuary. In either case, long-term data that testthe pattern of repeated progression of seasonal assemblagesare largely unavailable.
In this paper, we consider larval supply and the temporalstability of estuarine fish assemblages by examining an ich-thyoplankton assemblage in the polyhaline portion of a south-ern New Jersey, U.S.A., estuary with weekly sampling over6 years. In this estuary, as in others, each year of observa-tions involves a new year-class of all species involved; thus,the influence of interannual variation in larval supply can beclearly identified. If the larval fishes occur in repeated, pre-dictable sequence, we may infer that larval supply maystabilize an annual pattern of species abundances. Alterna-tively, if the seasonal sequence of species occurrence variesfrom year to year, or if larval absolute and rank abundances
Can. J. Fish. Aquat. Sci.56: 222230 (1999) 1999 NRC Canada
Received April 27, 1998. Accepted October 8, 1998.J14559
D.A. Witting 1 and M.P. Fahay. NOAA/National MarineFisheries Service, James J. Howard Marine SciencesLaboratory, 74 Magruder Rd., Sandy Hook, NJ 07732, U.S.A.K.W. Able. Marine Field Station, Institute of Marine andCoastal Sciences, Rutgers University, 800 Great Bay Blvd.,c/o 132 Great Bay Blvd.,Tuckerton, NJ 08087-2004, U.S.A.
1Author to whom all correspondence should be addressed.e-mail: email@example.com
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vary dramatically from year to year, we can infer that larvalsupply is a source of interannual variation in the seasonalprogression of species.
Study areaThe Great Bay Little Egg Harbor estuarine system (Fig. 1) is
polyhaline and shallow (average depth 1.7 m). It comprises adrowned river valley (Mullica River), embayment (Great Bay), andadjacent barrier beach estuary (Little Egg Harbor). A natural inlet(Little Egg Inlet) is the primary source of ocean water entering thisestuary. Several thoroughfares or creeks, including Little Sheeps-head Creek, run through a peninsula and serve to connect GreatBay and Little Egg Harbor. This estuary shares many characteris-tics with other estuaries in the Middle Atlantic Bight including abroad seasonal temperature range (2 to 28C) and a moderatetidal range (about 1 m) (Able et al. 1992). The fauna of residentand migratory fishes is enriched by northern and southern migrantspecies (Able and Fahay 1998).
All samples of larval and early juvenile fishes, hereafter referredto as larvae, were collected from a bridge that spans Little Sheeps-head Creek (Fig. 1). The bridge is located 3 km from the creekmouth, 2.5 km from Little Egg Inlet. Water depth at the samplinglocation is about 4 m. Atlantic Ocean water flows into the estuarythrough Little Egg Inlet during flood tides, some directly into themouth of Little Sheepshead Creek.
Sampling protocolWe collected samples weekly by suspending a 1-m-diameter
(1-mm mesh) plankton net from the bridge during night flood tides(referred to hereafter as a deployment). Five 30-min deploymentsof two nets, one at the surface and one near the bottom, produced atotal of 10 deployments per sampling date beginning in February1989. Between May 1990 and July 1991, we made three deploy-ments of two nets at these same depths, and between August 1991and November 1994, we made three deployments of one net half-way between surface and bottom (midwater). There was no shift inspecies abundance (relative, absolute, or rank) in the year that wechanged from paired surface and bottom deployments to a singlemidwater deployment. To estimate the volume of water sampled,we fixed a General Oceanics flowmeter in the mouth of the net.We made a total of 42 deployments without flowmeters between 1and 17 February 1989 and 1309 deployments with flowmetersbetween 20 February 1989 and 30 November 1994. We did notinclude samples from deployments that were made without flow-meters in any analyses of density. We sampled an average of 401 m3
(150 m3 SD) of water in each tow, and a small fraction (5%) of de-ployments sampled less than 100 m3. At the beginning and end ofeach sampling effort, we measured surface water temperature witha field thermometer and salinity with a refractometer. To determinelong-term interannual variation in temperature, we also incorpo-rated a data set of daily water temperatures recorded in the estuaryspanning 18 years (Able et al. 1992).
After each deployment, we sorted all samples in the laboratoryby placing small portions of the samples in shallow pans and re-moving all fish, which were then preserved in 95% ethanol. Wethen identified, measured, counted, and assigned a notochord flex-ion stage to each preserved fish. We used notochord flexion tocharacterize the onset of the transition from larval (preflexion) tojuvenile (postflexion) life history stages. Lengths were also used toindicate ontogenetic state; however, we consider flexion stage tobe a better indicator than length due to the potentially confoundingeffects of shrinkage that may occur due to preservation. We consid-ered larvae to be in the preflexion stage if the notochord wasstraight at the caudal tip, flexion stage if the notochord flexed dor-sally at the caudal peduncle and hypural formation had begun, andpostflexion stage if the hypural plate was fully formed.
There were a few taxonomically confusing forms. For example,Ammodytesmay be represented by two species, but was most likelyAmmodytes americanusbecause of the estuarine sampling location(Nizinski et al. 1990). In addition, we did not resolve the identifi-cation of two species of goby (Gobiosoma boscand Gobiosomaginsburgi) until the third year of sampling (Duval and Able 1998);thus, we combined them asGobiosomaspp. for the 1989 and 1990samples.