Habitat-based assessment of lobster abundance: A case study of an oil spill

The American lobster Homarus americanus is usually associated with rocky su bstrate that provides or can be modified into shelter and that may be an es sential habitat to early benthic-phase juveniles. The dependence on shelter -providing habitat not only makes possible the definition of essential habi tat for lobsters but also permits the assessment of abundance based on the areal extent of habitat. Here, we describe such a habitat-based assessment, performed in response to an oil spill on the coast of Rhode Island, USA. R esults from a side-scan sonar survey performed after the spill indicated th at the amount of lobster habitat affected by the oil was approximately 9.8 km(2) along nearly 15 km of coastline. Postspill lobster density ranged fro m 0.24 lobsters m(-2) in the impact region to 1.63 lobsters m(-2) in the co ntrol region. Qualitative (map contours of lobster density) and quantitativ e (statistical tests) approaches suggested a significant effect of the spil l had been detected by our sampling. An estimate of the total number of lob sters killed was required to scale restoration efforts. We calculated the t otal number of lobsters in the area by overlaying contours of lobster densi ty on a habitat map generated by side-scan sonar, then multiplying the dens ity of lobsters in each contour interval by the area of appropriate lobster habitat (cobble and boulder) in the contour interval. To calculate loss, w e subtracted postspill abundance from prespill abundance. Prespill density was estimated to be 1.76 m(-2), which is an adjusted average of airlift sam ples taken at six Rhode Island sites four months prior to the spill. Calcul ations of loss based on habitat-specific density estimates were adjusted to reflect undersampling. The loss was estimated to be to be 9.0 infinity 10( 6) lobsters. Variability associated with this loss estimate is large; 95% c onfidence intervals estimated that between 6.7 infinity 10(6) and 15.6 infi nity 10(6) lobsters were lost. The calculated loss was very sensitive to ch anges in prespill density estimates; a change of 0.1 lobsters m(-2) resulte d in a change of 0.75-0.9 infinity 10(6) lobsters lost. Habitat-based asses sment of lobster population size is possible but requires detailed habitat maps and accurate density estimates. Natural variability and sampling Limit ations give such assessment a wide range of possible values. Nevertheless, the airlift sampling technique, together with side-scan sonar maps of habit at, could provide a powerful tool for estimating the abundance of inshore l obsters.