The stomachs from YOY winter flounder, tautog, and Atlantic tomcod used in several of the caging growth experiments (1996, 1997, 1998) were removed and the contents were identified. Afterwards, the contents were dried and weighed to examine stomach fullness. This procedure only revealed the diet offishes at the endof the experiment but it probably represented the diet of the fishes during the entire ten day feeding experiments.

The types of prey consumed by winter flounder caged under piers, at pier edges, and in open water were similar and benthic organisms comprised the majority of the stomach contents. Interestingly, winter flounder caged under piers had some food in their stomachs but growth under piers was negative. Thus, it seemed that their restricted energy intake was not sufficient to meet their metabolic expenditures. Principal prey items of winter flounder caged at all locations were harpacticoidcopepods andgammarid amphipods.

Polychaetes, isopods, barnacles, ostracods, and brachiopods were also found in the stomachs of some fish caged at edges and in open water, but were absent from the stomachs of under-pier fish. Mean stomach content dry weights were generally lower under piers (X = 0.07 mg ± 0.13) than in open water (X = 0.34mg ± 0.46),whichisconsistentwith lower growth under piers compared to edges or outside.

Results were similar among caged tautog. Tautog also consumed primarily benthic organisms, though identification ofstomach contents in this species was more difficult due to the grinding of food items with the pharyngeal teeth (Olla et al., 1974). Most of the contents appeared to be harpacticoid copepods and mysids, though am-phipods were occasionally found in the stomachs offishes caged outside of the pier. Tautog caged outside of the pier generally had higher stomach content dry weights (X = 0.24mg ± 0.54) than of those caged under the pier (X = 0.06 mg ± 0.12) andlower stomach weights were probably directly related to poor growth of tautog under piers.

The diets of caged Atlantic tomcod were very similar to those of caged winter flounder and tautog; Atlantic tomcodconsumedbenthic prey organisms (see Metzger et al., 2001 for a more complete description). In fact, we did not find a single planktonic prey item in the stomachs of the dissected fish even though this has been reported in other studies (Grabe, 1978). Principal prey items for caged tom-cod were harpacticoid copepods and amphipods, though we also found isopods, nematodes, invertebrate eggs, saltwater mites, and a polychaete. Like winter flounder and tautog, Atlantic tomcod caged under the pier had a lower mean stomach content dry weight (X = 0.34mg ± 0.77) than fish caged at the edge (X = 0.99 mg ± 1.3) or outside of the pier (X = 1.01 mg ± 1.13), again probably contributing to observed lower growth rates. It appeared that growth of Atlantic tomcod under piers was not explained by differences in diet. It seemed more likely that our first hypothesis, that Atlantic tomcod could forage more efficiently in low light than winter flounder or tautog, was correct.

Recall that at the conclusion of the growth experiments we determined that the growth rates of all three test species, winter flounder, tautog, and Atlantic tomcod were depressed under piers relative to edges or open water. Since the above experiments revealed that all three of these species exploited the same food source, it could be that the general depression in growth rates under piers was due to lower abundances of benthic prey under piers compared to edges or outside. Since this hypothesis remained untested, we attempted to quantify the benthic prey organisms in the sediments around a pier to determine whether prey levels under piers were depressed.

0 0

Post a comment