Abstract
Forage species form a link between trophic levels in the marine ecosystem, facilitating energy flow from primary producers to apex predators. Ecologists and fishery biologists study forage species and their energy content in order to make predictions about predator populations using bioenergetics models. It is important to obtain energy values specific to the species, life stage, geographic location, and time of year to enhance these model predictions. Eight forage species from the Northwest Atlantic were analyzed for proximate composition (water, lipid, protein and ash) and energy content: Alewife(Alosa pseudoharengus), Atlantic herring (Clupea harengus), butterfish (Peprilus triacanthus), silver hake (Merluccius bilinearis), sand lance (Ammodytes sp.), Atlantic mackerel (Scomber scombrus), Northern shortfin squid (Illex illecebrosus), and Longfin squid (Loligo pealeii). These were collected from Georges Bank, Gulf of Maine, Southern New England and Mid-Atlantic Bight regions. Three objectives were examined:1) the relationship between percent dry weight and energy density, 2) the extent to which energy density of forage species depends on the body mass, and season, latitude, and year of collection, and 3) changes in forage species energy density in the study area over the last three decades. Percent dry weight and energy density were highly correlated for all species, indicating that percent dry weight provides a tool for estimation of energy density. Linear models were constructed for each species to evaluate to what extent abiotic and biotic factors influenced energy content. Body mass influenced the energy density of all species except Northern shortfin (Illex) and Longfin (Loligo) squid species. Latitude and season influenced the energy content of all study species except Loligo squid. Collection year influenced the energy content of alewife and Atlantic herring, and finally, the effects of latitude and season were found to interact for butterfish, Atlantic mackerel, and Illex squid. Mean energy densities were compared to those from previous studies to examine the extent of interdecadal changes in energy density. Although analyses were limited by sample sizes in the historic studies, it appeared that there may have been changes in the average energy densities of the study species, which indicates that there may have been a shift in forage species energy content over time in the Northwest Atlantic.