Geospatial analysis of habitat use by silver hake Merluccius bilinearis in the Gulf of Maine

Abstract

Three theories of habitat use proposed for marine fishes, the constant density model (CDM), the proportional density model (PDM), and the basin model (BM), make contrasting pre- dictions of how the range, local density, and fitness change as population size changes. We tested model predictions using a survey of silver hake Merluccius bilinearis conducted by the National Marine Fisheries Service every spring and fall. Data on fish length, sex, weight, and reproductive status are collected. Despite a 50% population reduction over the time period of the survey, the geographic area occupied by silver hake did not change significantly. Local density in preferred habitat remained a constant proportion of the entire population. Fitness (as indicated by individ- ual condition) varied over a fine spatial scale, reflected in a pattern of spatial autocorrelation of 10s to 100s of kilometers. Fitness did not exhibit density dependence, except in one case, females in the fall, probably due to the higher energy needs of the females. The results indicate that silver hake conform to the PDM, a site-dependent model, and yet we found some evidence of density dependence in our measure of fitness. Site dependence may work in concert with density depend- ence to regulate population size. Sites silver hake depend on may be the spawning areas along the coast, which may already be in legislatively protected areas. Our study illustrates that an under- standing of habitat use is important for crafting conservation or management measures that lead to maintenance of a stable population.