Decadal trends in age structure and recruitment patterns of ocean quahogs Arctica islandica from the Mid-Atlantic Bight in relation to water temperature

Occan quahogs (Arctica islandica) are long-lived bivalves. Distributionl patterns and biology of ocean quahogs ill the Mid-Atlantic Bight (MAB) off the cast coast of North America are directly related to bottom water temperatures. We examined long term recruitment patterns for ocean quahogs across temporal (decadal) and spatial (latitudinal. bathymetric) scales Using a spatially defined (Long Island Sound to Chesapeake Bay mouth) population encompassing a broad size (age) range of animals that had not yet recruited to the commercial fishery [<80 mill shell length (SL)], An age-at-length relationship for quahogs less than 80 mm SL is described using a power function. Quahog age did not vary significantly with depth or region, nor were any interaction terms between age and length with depth or region significant. An age-length key was developed for ocean quahogs to generate age frequencies for each station. Principal components analysis (PCA) oil the resulting age-frequency distributions standardized per low enabled construction of characteristic age-frequency distributions for similar stations identified by the PCA factor scores. These characteristic age-frequency distributions identified quahog cohorts with modal ages corresponding to recruitment during the 1948-1950. 1954-1959 1972-1980 and 1978-1983 time periods. Observed recruitment patterns in MA B ocean quahogs are strongly related to bottom water temperature patterns. Years it) Which the number of months with water temperatures averaging 6 degrees C to 10 degrees C exceeds the number of months with water temperatures less than 6 degrees C by at least two months are also years that contriute Strongly to the modal year classes in the population age-frequency distributions. In general. years with above average bottom water temperatures during January, February. and March lend to produce year classes that are distinct in the age-frequency distributions front the MAB (quahog populations. The observed time series of quahog recruitment Operates at a different time scale than stock surveys and most estimates of fishery dynamics. The 50-60-y lag between quahog recruitment to the benthos and recruitment to the fishery presents challenges for fishery forecasting in that changes in adult biomass and subsequent effects oil stock-recruit relationships will only become evident oil this this scale.