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Habits of Juvenile Fishes in Two Rhode Island Estuaries

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The basic purpose of this work was to gain information on the possible role of some Rhode Island estuaries as nursery grounds for young migrant and resident species of fishes.

The areas selected were, the lower Pettaquamscutt River and the lower Point Judith Pond, both in the town of Narragansett, Rhode Island. The seining operations were carried through summer and early fall of 1962 when these estuaries are heavily used as nursery grounds. Major features of the occurrence, abundance and distribution of young fishes were deduced by examining samples from seine hauls. Thirty-six species were recorded from the lower river while only twenty-four species occurred in the lower pond. The abundance of fishes rose with a rise in temperature and declined with decreasing temperature, but no correlation was observed between maximum temperature and maximum number of individuals occurring at any time. The number of species and the abundance of individual fish were highest at the seaward station (Sta. II) in the lower river. Among the selected species, the abundance of Menidia menidia was two to three times higher at Middle Bridge (Sta. II) than at any other station. The behavior of Pseudopleuronectes americanus, found primarily at seaward stations, and the migrant species Brevoortia tyrannus observed at landward stations, is discussed.

The species P. americanus grew at the rate of 10 mm per month, but exhibited no variation in growth in the two estuarine systems. The populations of B. tyrannus from the lower Pettaquamscutt River had a growth rate that was almost twice that of populations in the lower Point Judith Pond. The growth rate of these species in Rhode lsland waters compare favorably with similar data from other studies. The juvenile M. menidia demonstrated a higher rate of growth at seaward stations in both the areas, especially in the lower river.

Forty-three types of prey organisms belonging to diverse taxonomic groups were identified from stomach contents of P. americanus and thirty-nine types were noted in the gut contents of M. menidia. Amlysis of the degree of fullness indicated markedly high percentage of full stomachs in the two study areas. However the degree of fullness was comparatively less in fish occurring in the lower pond. The scarcity of food in the lower pond, apparently forced M. menidia (51-80 mm) to feed upon phytoplankton as a substitute food or “forced diet”. In P. americanus and M. menidia a change in diet was noted with change in size. The taxon, B. tyrannus, which depended upon phytoplankton and suspended organic matter, did not show any change in food with change in body size.

While no effective predation was observed, an infection by the sporozoan parasites, Glugea hertwigi, was marked in both Osmerus mordax and P. americanus. Low catches of P. americanus were perhaps due to higher infection.

A comparison of the parameters of abundance, growth and food habits reveal that the two estuarine systems are suitable nursery grounds, and that the lower river is a more favorable nursery than the lower pond.

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