Implications of recent increases in catches on the dynamics of Northwest Atlantic spiny dogfish ( Squalus acanthias)

Abstract

US commercial landings of spiny dogfish have increased five-fold since 1987 to over 22 000 mt in 1993. Over 95% of the landings consist of mature females ≥80 cm in length. Minimum discard estimates for 1993 suggest an additional 25 000 mt of dogfish were discarded, of which 14 000 mt were killed. LOWESS-smoothed minimum swept-area biomass estimates reveal a five-fold increase in abundance over the last 30 years to about 650 000 mt in 1993. There is no evidence, however, of a continuing increase in the fishable stock (i.e., ≥80 cm in length) since 1990. Mean lengths of dogfish in commercial landings and research survey catches have decreased in the last five years, and fraction of females >80 cm in the survey has declined markedly. Change-in-sex-ratio estimators were used to derive sex-specific F values. Results suggest a five-fold increase in F on females from 1991 to 1993 to levels in excess of 0.25 per year. Commercial fishery information, research survey data, and life history parameters from the literature were used to develop a size- and sex-structured equilibrium model for dogfish. Using the concept of pups per recruit, preliminary biological reference points for fishing mortality were derived. With a minimum size limit of about 80 cm, fishing mortality rates ( F) in excess of 0.2 would lead to the gradual decline of the spiny dogfish stock. While current minimum biomass estimates are high, mature females may already be overexploited. In view of the delayed maturation, low birth rates, and longevity of this species and experiences in shark fisheries worldwide, plans to increase exploitation rates should proceed cautiously to avoid rapid depletion of the adult stock.