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EFFECTS OF A MUNICIPAL PIER ON GROWTH OF YOUNG-OF-THE YEAR ATLANTIC TOMCOD (MICROGADUS TOMCOD): A STUDY IN THE HUDSON RIVER ESTUARY.

CHARLES V. METZGER [1,2]

JANET T. DUFFY-ANDERSON [1,3]

KENNETH W. ABLE [1]

ABSTRACT: The effects of a large, municipal pier on the growth of young-of the-year (TOY) Atlantic tomcod (Microgadus tomcod) was investigated in two 10-d experiments conducted in the Hudson River estuary. Fish (42-75 mm TL) were confined in benthic cages along a transect that ranged from open water to underneath a pier and consisted of three stations: 40 m beyond the pier edge in open water (+40 m), the pier edge (0 m), and 40 m underneath the pier (-40 in). In both experiments, all fish grew significantly more than in controls. These results contrasted with previously published reports that indicated some species of juvenile fishes lose weight in similar experiments under piers. Mean instantaneous growth rates in weight (Gw[sup.-1]) of Atlantic tomcod in the first experiment were +0,02 [+ or -] 0.004 [d.sup.-1] under the pier, +0.04[+ or -] 0.006 [d.sup.-1] at the pier edge, and +0.03 [+ or -] 0.005 [d.sup.-1 outside of the pier. Results were similar in the second experiment with means of +0.02 [+ or -] 0 .002 [d.sup.-1], +0.03 [+ or -] 0.002 [d.sup.-1], and +0.03 [+ or -] 0.003 [d.sup.-1], underneath, at the edge, and outside of the pier, respectively. These data demonstrate that TOY Atlantic tomcod can grow in under-pier habitats, though there is a trend toward lower growth compared with edge or open water habitats. Analyses of stomach contents indicate that Atlantic tomcod consumed a diet of benthic invertebrates (harpacticoid copepods, amphipods) and no significant differences in total stomach content dry weights were detected across the transect. Benthic core samples collected along the transect indicated a trend toward higher densities of prey [- or X] = 15 [+ or -3] prey [cm.sup-3] underneath the pier. Taken collectively data suggest that benthic prey are available for YOY Atlantic tomcod across the pier transect and that Atlantic tomcod can grow under piers although feeding and growth is reduced compared with adjacent edge and open water areas. Therefore, under-pier areas are probably sub-optimal habi tat for this species, a general observation that is consistent with other species examined.

KEY WORDS: Microgadus tomcod, Atlantic tomcod, Hudson River estuary, growth

INTRODUCTION

New York Harbor is a large, urban estuary that also serves as an important nursery area for a variety of juvenile fishes. Anthropogenic influences in the harbor have altered or destroyed most of the natural fish habitat (Squires,

1992), often replacing it with man-made structures such as shipwrecks, abandoned pile fields, bulkheads and piers. There is evidence that some of these man-made structures may be used as habitat by juvenile fish (Able et al., 1998), though it appears that not all structures are suitable for all species. Previous work has demonstrated that abundances of fishes and decapod crustaceans are depressed under large municipal piers (Able et al., 1998), and that growth rates of young-of-the-year (YOY) winter flounder (Pseudo-pleuronectes americanus) and tautog (Tautoga onitis) are negative (i.e., the fish lose weight) when caged under piers for extended periods (Duffy-Anderson and Able, 1999; Able et al., 1999). This has led to speculation that under-pier areas are poor-quality habitats for j uvenile fishes in general. However, at least two species of fish, the American eel (Anguilla rostrata) and the Atlantic tomcod (Microgadus tomcod) are repeatedly collected under piers (Stoecker et al, 1992). For example, the results of a trapping study in the lower Hudson River estuary indicate that over 25% of the total number of Atlantic tomcod collected during the early spring were obtained from under-pier sites and nearly 50% of American eels were found under piers (Able et al., 1998). In contrast, less than 1% of the total number of winter flounder and tautog were collected from under-pier sites during the same time period.

A variety of approaches have been.adopted in an attempt to assess habitat quality including species abundance (Szedlmayer and Able, 1996), species richness (Heck et al., 1995), species distribution (Sogard and Able, 1994; Jenkins and Sutherland, 1997; Jenkins et al., 1997), and fish guilds (Bond and Stephens, 1999). However, measurements of growth rate may be well-suited to examining habitat value among juveniles since growth is very rapid during this period and therefore readily quantffied (Able, 1999). For example, Sogard (1992) used growth rates to demonstrate that vegetated areas may be more suitable for growth of YOY tautog than open-water sites, and Rountree and Able (1992) used comparative growth rates to determine that salt-marsh creeks were important habitats for YOY summer flounder (Paralichthys dentatus). In the present study, we assessed the value of under-pier areas as habitat for YOY Atlantic tomcod, a species frequently collected from under-pier areas, by determining differences in instantaneou s growth rates among fishes caged under a pier, at a pier edge, and in the open water beyond a pier. We speculated that Atlantic tomcod may be better able to exploit darkened habitats than other species we have examined (tautog and winter flounder) because previous work has demonstrated that other gadids may have increased chemo- or mechanosensory abilities that supplement visual foraging techniques (Bardach and Case, 1965; Doing and Selset, 1980; Pearson et al., 1980). As such, under-pier areas may provide more suitable habitat for Atlantic tomcod than for other species.

METHODS

Target Species

Atlantic tomcod (Pisces: Gadidae) occur in western North Atlantic coastal waters from Labrador to New Jersey (Able and Fahay, 1998), with a resident population occurring in the Hudson River (Dew and Hecht, 1994). Adults spawn in December and January upriver from the salt-water intrusion. Larvae are transported downstream toward the mouth of the Hudson River estuary, including the vicinity of the study site where they assume a demersal existence (Dew and Hecht, 1994). Young-of-the-year individuals move back up river into less-saline water from April through July. In this system Atlantic tomcod rarely survive beyond their first year and it is believed that the majority of the Hudson River spawning stock is composed of YOY individuals.

Site Description

Marine and Aviation Pier 40 (Figure 1; 40[degrees] 44' N, 74[degrees] 01' W) was selected as the study site because of its accessibility and its previous use in studies of fish habitat quality (Able et al., 1998; Duffy-Anderson and Able, 1999). The pier is on the Manhattan side of lower New York Harbor, across from Hoboken, New Jersey (Figure 1). The pier measures approximately 250 mx 350 m and the top is made of solid concrete. The transect used in this study was established on the northern side of the pier and three stations were set up along the transect: 40 m beyond the edge of the pier in open water (+40 m), the pier edge (0 m), and 40 m underneath the pier (-40 m). Station depths along the transect ranged from 3--4.5 m.

Environmental Conditions

Water temperature ([degrees]C), salinity (0/00), and dissolved oxygen level (mg [L.sup.-1]) were recorded at regular intervals using dataloggers (Hydrolab Corp.). These were attached to concrete blocks to prevent movement, and were deployed at both the -40 m and +40 m stations. Additional dataloggers (OnSet Corp.) were fastened to deployment cages at the same two stations to record light levels hourly over the course of the experiments.

Relative net water movement was determined at the -40 m and +40 m stations by comparing plaster dissolution rates in three separate experiments. Plaster standards (Angradi and Hood., 1998) were prepared, molded, sanded to a constant initial weight (ing), and attached to petri dish lids using marine silicone. Eight standards were randomly chosen and attached to a concrete block and two replicate blocks were deployed at each station. Standards remained in the field between 24--48 h, depending on the experiment. Upon retrieval, they were removed and allowed to dry at room temperature for no less than 96 h, after which final weights were recorded.

We determined whether there were differences in benthic prey availability for fishes caged along the transect by coring the sediments at each transect station 3 d after the conclusion of the second growth experiment. Four replicate core samples (3.0 cm diameter, 2.0 cm depth) were collected at each transect station and immediately preserved in 4% formaldehyde. Samples were returned to the laboratory where the contents were stained with Rose Bengal, sorted, identified, and enumerated.

Prey organisms were pooled into broader taxonomic categories for data analyses. Taxa included Polychaetea, Oligochaeta, Maxillopoda, Malacostraca, Bivalvia, and Other, which included rare items including invertebrate eggs, fiatworms, and tunicate. Prey sizes were not measured.

Fish Collection, Size, Instantaneous Growth

Young-of-the-year Atlantic tomcod were collected from the lower Hudson River (40[degrees] 44' N, 74[degrees] 01' W) using benthia traps and were also seined from nearby Little Neck Bay, NY (40[degrees] 47' N, 73[degrees] 46' W) and Princes Bay, NY (40[degrees] 30' N 74[degrees] 12' W). Fish were returned to the laboratory, held in flow-through tanks for approximately 1 wk, and fed a diet of Artemia spp. nauplii (Great Salt Lake, Utah) and chopped clams. They were maintained on a 14 h light/10 h dark photoperiod, during which time the temperature and salinity levels in the laboratory ranged from 16--19[degrees]C and 21-250/00, respectively.

Two separate 10-d field experiments were conducted (May, June 1998). Individual fish sizes ranged from 42--70 mra SL for the first experiment and 52--75 mm SL for the second (Table 1). Two days prior to an experiment, the feeding regime was stopped to allow the fish to eliminate the contents of the gut. One day prior to deployment fish were weighed (mg) and placed in 1 L containers with flow-through mesh lids. Benthic cages used in these experiments were identical to those used previously to determine growth of winter flounder and tautog under piers (Duffy-Anderson and Able, 1999; Able et al., 1999). Briefly, cages were constructed of a welded steel frame (0.85 m x 0.85 m x 0.45 m), with a 3 mm mesh bag hung in the interior. The mesh retained the fish, but allowed passage of water, sediment, and potential prey items. Previous studies have demonstrated that tomcod are capable of feeding through the 3 mm mesh liner (Duffy-Anderson, unpublished data). On the day of an experiment, the fish were transported to th e study site in coolers filled with filtered seawater. Once at the pier site, one fish was randomly transferred to each cage and the cage was lowered to the bottom. During the first experiment, eight replicate cages were deployed to the +40 m station, seven cages to the 0 m station, and eight cages to the -40 m station. During the second experiment, five cages were deployed at each station. In both experiments, caged fish remained in the field for 10 d, after which the cages were retrieved and the fish were transported alive back to the laboratory for final weight determination.

Simultaneous with the above procedures, five fish were chosen at random during each experiment to serve as controls. These fish were held in the laboratory in fiberglass tanks in ambient filtered seawater without food for the duration of each field experiment. Weighing and handling procedures for control fish were identical to those for experimental fish.

Stomach Contents

Since YOY Atlantic tomcod are collected from waters beneath piers more frequently than are winter flounder or tautog (Able et al., 1998), we reasoned that differences in diet may support their ability to occur in this habitat. In order to determine the composition of the diet of YOY Atlantic tomcod we dissected the stomachs from all fish used in these experiments and identified the contents. Afterwards, the contents were dried in a 60[degrees]C drying oven and weighed in order to compare stomach fullness across habitat types. Taxonomic categories were identical to those used in benthic prey availability evaluations (see above). Again, prey sizes were not recorded.

Statistical Analyses

Differences in initial fish size were determined using two-way ANOVA design with transect location and experiment date as the two factors. Instantaneous growth rate in weight ([G.sub.w]) was calculated according to:

[G.sub.w] = (In [W.sub.f] - In [W.sub.i]) / t

where [W.sub.i] = initial weight prior to deployment, [W.sub.f] = final weight after experiment, and t = the duration of the experiment in days. Differences in instantaneous growth rates among fish caged at the three stations were analyzed using one-way ANOVA procedures. Tukey multiple comparison tests were used to show where differences existed, and inference was made at [alpha]= 0.05. Analyses of stomach content dry weights were conducted using the same procedure. Differences in prey type in the core samples and within the stomachs of the fish were determined using a non-parametric analysis of similarity (ANOSIM) design. In an ANOSIM analysis the test statistic (R) is used to evaluate the degree of separation between samples; R=1 if all replicate samples within a group are more similar to one another than to any replicates in other groups, R = 0 if similarities within sites and between sites are equal. R is generally evaluated on whether or not it is significantly different from zero.

RESULTS

Environmental Conditions

During the field experiments average daily temperatures at depth ranged from 14--19[degrees]C. Salinities ranged from 13--25%, and dissolved oxygen levels ranged from 4--6 mg [L.sup.-1] (Figure 2). Temperature, salinity, and levels of dissolved oxygen at the -40 m station during the second trial were not recorded properly by the datalogger, however we expect that those measurements followed closely with those at the +40 m station, as in the first trial. At-depth light intensities outside of the pier were 0.64 [+ or -] 1.3 lumens [m.sup.-2], while under the pier they were significantly reduced to 0.002 [+ or -] 0.004 lumens [m.sup.-2] (p [less than] 0.001). Light intensities were measured at the time of deployment and upon recovery for each experiment. Measurements were taken in the morning (at approximately 8 AM) and most days were relatively clear. Data collected from replicate deployments of plaster of Paris standards (n = 3) showed that greater mass loss occurred outside of the pier (X = 73 [+ or -] 0.69 g [d.sup.-1]) than underneath (X = 6.7 [+ or -] 0.43 g [d.sup.-1]), suggesting that water movement is slightly dampened under piers.

Analyses of the core samples indicated that the majority of the samples were made up of nematodes and foraminifera (X = 81%) with harpacticoid copepods comprising the next greatest percentage (14%). The remainder of the samples were made up of calanoid copepods and nauplii, oligochaetes, polychaetes, invertebrate eggs, ostracods, flatworms, saltwater mites, and bivalves. However, nematodes and foraminifera are not typically considered prey for YOY Atlantic tomcod (Grabe, 1978) so they were eliminated from further analyses. ANOSIM analysis revealed no significant differences in benthic prey type or prey distribution across the pier transect (R = 0.20), though lower abundances were noted outside of the pier (X = 6[+ or -]2 prey [cm.sup.-3]) than at the edge (X = 12[+ or -]4 prey [cm.sup.-3]) or underneath (X = 15 [+ or -]3 prey [cm.sup.-3]).

Fish Size and Instantaneous Growth

There were no significant differences in initial fish sizes among stations (p = 0.09) or between experiments (p = 0.72), and there was no significant interaction between the two factors (p = 0.06) (Table 1). Since body sizes were relatively similar among transect stations and between experiments, any differences in feeding noted were probably not influenced by differences in body size or date of experiment. Instead, it is more likely that differences were the result of caging location.

Young-of-the-year Atlantic tomcod caged under piers grew in both 10-d experiments, though instantaneous growth rates under piers were lower than at edges or in open water (Figure 3). In the first experiment, mean growth rates under the pier were +0.02 [+ or -] 0.004 [d.sup.-1], at the edge were +0.04 [+ or -] 0.006 [d.sup.-1], and outside were +0.03 [+ or -] 0.005 [d.sup.-1]. For the second experiment, growth rates were similar, with rates of +0.02 [+ or -] 0.002 [d.sup.-1], +0.03 [+ or -] 0.002 [d.sup.-1], and +0.03 [+ or -] 0.003 [d.sup.-1] determined underneath, at the edge, and outside of the pier, respectively. Significant differences in instantaneous growth rates in the first experiment were detected between fish caged under the pier and those caged at the edge (p [less than] 0.001), while in the second experiment, under-pier fish grew significantly less than fish caged at both the edge and outside stations (p [less than] 0.001). Growth of laboratory-starved control fish was negative in both experiment s (X = -0.02 [d.sup.-1] and -0.02 [d.sup.-1], respectively) and was significantly different from all field-deployed animals.

Stomach Contents

ANOSIM analyses of stomach contents of fish from each experiment revealed that there were no significant differences in type of prey consumed across the transect (R = 0.04, R = 0.06 for the first and second experiments, respectively. R not significantly different from 0). Regardless of station, principal prey items consisted of harpacticoid copepods and amphipods (whole and in part), though isopods, nematodes, invertebrate eggs, polychaetes and salt water mites were also found in the stomachs of some fish. Atlantic tomcod caged under the pier had lower mean stomach content dry weight (mg) than fish caged at the edge or outside of the pier (Figure 4), though significant differences were only detected in the second experiment (p = 0.02).

DISCUSSION

The results of these experiments show that YOY Atlantic tomcod can grow under piers, though at reduced rates compared to edge or openwater habitats. This result is in contrast with previous reports that demonstrate that juvenile winter flounder and tautog show negative growth rates when caged under piers ([G.sub.w]= -0.015 and -0.023 [d.sup.-1], respectively) in New York Harbor (Duffy-Anderson and Able, 1999; Able et al., 1999; Duffy-Anderson and Able 2001).

We suggest two hypotheses to explain positive growth of YOY Atlantic tomcod under piers. First, Atlantic tomcod could exploit a different food source than the fish species we examined previously. Winter flounder and tautog consume primarily benthic organisms (Pearcy, 1962; Grover, 1982), but Grabe (1978) showed that Atlantic tomcod of the same size range as those used in this study primarily consumed planktonic copepods. However, we dissected the stomach contents of all fish used in this study and found no planktonic prey items in the stomachs of any fish. In fact, the diets of these fish were remarkably similar to the diets of YOY winter flounder and tautog. Therefore, it seems unlikely that differences in growth rate between Atlantic tomcod, winter flounder and tautog caged under piers could be explained by differences in diet.

A second hypothesis for growth of Atlantic tomcod under piers is that these fish may possess enhanced chemo- or mechanosensory acuities relative to winter flounder and tautog that allow them to better forage in low light conditions. Earlier we proposed that negative growth rates among winter flounder and tautog caged under piers may have been related to reduced light levels under piers, potentially limiting the ability of visually-feeding fishes to capture and consume prey (Duffy-Anderson and Able, 1999). However, the ability of fishes to feed in darkness is species-specific. Atlantic herring, Clupea harengus (Batty et al., 1986), roach, Rutilus rutilus (Diehl, 1988), and American eel, A. rostrata (Tesch, 1977), are all capable of feeding efficiently in darkness, while the ability of weakfish, Cynoscion regalis (Grecay and Targett, 1996), perch Perca fluviatilis (Diehl, 1988), and lined seahorse, Hippocampus erectus (James and Heck, 1994) are all compromised in low-light conditions. Very early work has shown that Atlantic tomcod rely on additional sensory cues (besides vision) to locate and consume prey (Herrick, 1904), and, like other gadids, the presence of a chin barbel may provide an additional mechanism for localizing food items (Pearson et al., 1980; Brawn, 1969). Interestingly the American eel, another species typically found under piers (Able et al., 1998), has a well-developed olfactory system (Tesch, 1977), which may be the reason it too can exploit under-pier habitats. We hypothesize that under-pier areas may be used as a habitat by a few select species, perhaps those with supplementary sensory systems, while simultaneously being inhospitable to a variety of other estuarine species.

While YOY Atlantic tomcod grow underneath piers, their growth potential seems to be higher at the pier edge and outside of the pier. Light deprivation under piers is likely to interfere with the visual foraging mechanisms of Atlantic tomcod, and we suspect that this may be the source of the lower observed growth rates and the reductions in stomach fullness under the pier. Therefore, while it appears that Atlantic tomcod are able to utilize under-pier habitats more effectively than some other fish species, under-pier areas are probably not optimal habitats for them. It is more likely that they use under-pier areas intermittently, perhaps sacrificing the fastest rates of growth for increased refuge from predation.

It should be noted that American eels frequently occur under piers (Able et al., 1998), making them a potential threat to cryptic under-pier juvenile Atlantic tomcod. It is unknown whether tomcod make up a significant portion of the diet of American eels, though other studies indicate that eels primarily consume macroinvertebrate taxa (Angermeier, 1992; Lookabaugh and Angermeier, 1992; Denoncourt and Stauffer, 1992). Therefore, eels may be less likely to prey on juvenile tomcod seeking refuge under piers. Another potential under-pier predator is the striped bass (Morone saxatilis), which may move into and out under-pier areas relatively quickly (Duffy-Anderson and Able, 2001). However, recent evidence indicates that striped bass do not prey on Atlantic tomcod (Dunning et al., 1997) making them an unlikely predator threat as well. Accordingly, darkened piers may offer an adequate predator refuge from at least these two major predator species.

Construction and restoration of municipal piers continues in the Hudson River and in other urban estuaries. Therefore, a more complete understanding of their effects on a variety of fish species is increasingly important. We have shown that growth of YOY Atlantic tomcod is positive when caged at three transect locations, under the pier, at the pier edge, and outside of the pier. It seems that although piers may be detrimental to the growth of some fish species (winter flounder and tautog), others, such as Atlantic tomcod, may be better able to exploit them as habitat. However, because growth rates and stomach content weights of YOY Atlantic tomcod caged under the pier were lower that those caged at the edge or outside, we suggest that under-pier areas are sub-optimal habitat even for species that are better able to utilize them.

ACKNOWLEDGMENTS

Thanks to J. Liming and D. Vivian for field assistance. D. Vivian and L. Rear helped with identification of invertebrates. C. Nieder and J. Waldman provided comments on an earlier draft of this manuscript. Support for this project was provided to Metzger by the 1998 Tibor T. Polgar Fellowship program, Hudson River Foundation, and by a grant provided to Able and Duffy-Anderson by the Hudson River Foundation, and the Rutgers University Institute of Marine and Coastal Sciences (IMCS). This is IMCS contribution number 2001-22.

(1.) RUTGERS UNIVERSITY, MARINE FIELD STATION, INSTITUTE OF MARINE AND COASTAL SCIENCES TUCKERTON, NEW JERSEY 08087

(2.) ROY F. WESTON, INC., EDISON, NEW JERSEY 08837

(3.) ALASKA FISHERIES SCIENCE CENTER, NOAA/NMFS SEATTLE, WASHINGTON 98115

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Table 1.

Standard lengths [+ or -) one standard deviation, number of replicates,
and percent recovery of Atlantic tomcod used in field experiments
conducted in the Hudson River estuary in May and June 1998.


EXPERIMENT LOCATION INITIAL FISH LENGTH NUMEER OF RECOVERY
 (MM SL) REPLICATES

May 1998 Under Pier 55.7[+ or -)6.6 8 75%
 Pier Edge 54.3[+ or -)6.2 7 100%
 Outside Pier 52.4[+ or -)7.7 8 75%
 Control 59.1[+ or -)5.6 8 100%

June 1998 Under Pier 62.5[+ or -)6.7 5 100%
 Pier Edge 57.0[+ or -)3.6 5 100%
 Outside Pier 59.0[+ or -)5.1 5 80%
 Control 59.3[+ or -)4.3 5 100%
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Author:METZGER, CHARLES V.; DUFFY-ANDERSON, JANET T.; ABLE, KENNETH W.
Publication:Bulletin of the New Jersey Academy of Science
Geographic Code:1U2NY
Date:Mar 22, 2001
Words:5086
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