Habits of Juvenile Fishes in Two Rhode Island Estuaries

T h e basic purpose of this work was t o gain information o n the possible role o f some Rhode Island estuaries as nursery grounds f o r young migrant and resident species of fishes. T h e areas selected were, t he lower Pettaquamscutt River and the lower Point Judi th Pond, both in the t o w n o f Narragansett, Rhode Island. T h e seining operations were carried through summer and early fall of 1962 w h e n these estuaries are heavily used as nursery grounds. Major features o f t h e occurrence, abundance and distribution of young fishes were deduced by examining samples from seine hauls. Thirty-six species were recorded f r o m t h e lower river while only twenty-four species occurred in the lower pond. T h e abundance o f fishes rose with a rise in temperature and declined with decreasing temperature, but n o correlation was observed between max imum temperature and max imum number of individuals occurring at any time. T h e number o f species and the abundance of indiuidual f ish were highest at t he seaward station (Sta. 11) in t h e lower river. A m o n g t h e selected species, t he abundance o f Menidia menidia was t w o t o three times higher at Middle Bridge (Sta. 11) than at any other station. T h e behavior of Pseudopleuronectes americanus, found primarily at seaward stations, and the migrant species Brevoortia tyrannus observed at landward stations, is discussed. T h e species P. americanus grew at t he rate of 10 mm per month, but exhibited n o variation in growth in the t w o estuarine systems. T h e populations of B. tyrannus f rom the lower Pettaquamscutt River had a growth rate that was almost twice that of populations in the lower Point Judi th Pond. T h e growth rate of these species in Rhode lsland waters compare favorably with similar data f rom other studies. T h e juvenile M. menidia demonstrated a higher rate o f growth at seaward stations in both the areas, especially in the lower riuer. Forty-three types of prey organisms belonging t o diverse taxonomic groups were identified f r o m stomach contents of P. americanus and thirtynine types were noted in the gut contents o f M . menidia. Amlys i s of t he degree of fullness indicated markedly high percentage of full stomachs in the t w o study areas. However the degree of fullness was comparatively less in fish occurring in the lower pond. T h e scarcity of food in the lower pond, apparently forced M . menidia (51-80 mm) t o feed upon phytoplankt o n as a substitute food or “forced diet”. Im P. americanus and M. menidia a change in diet was noted with change itz size. T h e taxon, B. tyrannus, which depended upon phytoplankton and suspended organic matter, did not show any change in food with change in body size. W h i l e no effective predation was observed, an infection by the sporozoan parasites, Glugea hertwigi, was marked in both Osmerus mordax and P. americanus. Low catches o f P. americanus were perhaps due t o higher infection. A comparison o f t he parameters of abundance, growth and food habits reveal that t he t w o estuarine systems are suitable nursery grounds, and that t he lower river i s a more favorable taursery than the lower pond.