Implications of expanding thermal habitat to settlement-based forecasts of American lobster landings

The American lobster, Homarus americanus, fishery is the most valuable single-species fishery in the USA and Canada, and more than 90% of the USA share comes from the Gulf of Maine (GoM). New recruits to the fishery comprise approximately 85% of annual landings, and therefore landings are a reasonable indicator of the productivity of the fishery. The American Lobster Settlement Index (ALSI) was established in 1989 to record the interannual variability and overall trends in year class strength at the time larvae settle to the seabed, some 6-9 years before they enter the fishery. Widespread recent downturns in GoM settlement densities, as measured at shallow water monitoring sites, have raised concerns over future declines in landings. However, it is possible that the ALSI alone may underestimate year class strength if the area of suitable nursery habitat has expanded in a warming climate. Previous studies indicate lobster larvae prefer to settle where temperatures exceed ~12°C. Until recent years much of the eastern GoM has not exceeded this temperature. The availability of habitat above this thermal threshold, in combination with the ALSI, may give better estimates of year class strength. In our analysis we use modeled bottom temperatures from the Finite-Volume Community Ocean Model (FVCOM) to evaluate yearly changes in the area of seabed above the 12° C threshold during the settlement season for 12 sub-regions of the GoM. Forecasts using the product of ALSI and the expanding area of thermal habitat translate to less severe declines in landings than would predicted on the basis of ALSI alone. Although not all factors contributing to variable fishery recruitment can be accounted for, incorporating thermal habitat with ALSI may provide more realistic estimates of recruitment to the American lobster fishery.

Faculty Mentor: Damian Brady