Seaward Stations Research Articles

The relationship between farmed mussels,Perna canaliculus, and available food in Pelorus-Kenepuru Sound, New Zealand, 1983–1985

Condition index [CI = (meat weight × 100)/(total weight − shell weight)] of green mussels grown on longlines was monitored monthly for 2 years at 12 commercial farms in Pelorus-Kenepuru Sound. Water temperature and salinity, total paniculate matter, chlorophyll a and paniculate carbon were recorded at approximately 2-monthly intervals at six of the 12 CI stations. CI showed an annual cycle (high in autumn/winter/spring and low in summer) with year-to-year differences in amplitude, and a regional pattern (lowest at the most seaward stations, highest towards the upper reaches of the Sound). The environmental data paralleled the condition index data by showing gradients along the length of the Sound. Anomalies in the regional patterns resulted from localised environmental conditions occurring in major embayments off the main channel. On an industry-wide scale abiotic factors temperature and salinity) were more highly correlated with mussel condition than factors measuring available food (chlorophyll a, carbon and participates). However, at lower ambient food levels changes in food concentration were strongly associated with changes in CI. A food threshold, at about 200 μg C l−1, was identified, below which CI declined significantly. The variation in condition between years, months, and farms represents differences in meat yield which are economically significant.

Ecological observations on the commercial sand crab, Portunus pelagicus (L.), and its parasite, Sacculina granifera Boschma, 1973 (Cirripedia: Rhizocephala)

Abstract. Sacculina granifera was found in 12% of commercial sandcrabs, Portunus pelagicus, in Moreton Bay irrespective of sex. Although male and female crabs were randomly distributed about Moreton Bay, egg bearing females were most common at the seaward station. This and the distribution of epizoic barnacles on gills and carapace suggest the sexes school separately. The increased prevalence of barnacles on the carapace of infected crabs indicates Sacculina inhibits moulting. The distribution of interna infections in small crabs towards south and west and externa infections in large crabs in the easterly, seaward stations suggests Sacculina preferentially attacks young crabs as they move inshore and then induces crabs to behave like ‘berried’ (i.e. egg bearing) females by moving seaward as they grow. Morphologically female crabs are little changed by Sacculina, but the males show considerable modification which is reflected most accurately in the shortening of the chelar propodus to proportions similar to normal females. Infected crabs are sterile and internally the hepatopancreas becomes green rather than tan. Parasitized crabs were seen to groom their externae as ‘berried’ females groom their egg masses.

Habits of Juvenile Fishes in Two Rhode Island Estuaries

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