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

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 fema les greater than or equal to 80 cm in length. Minimum discard estimates for 1993 suggest an additional 25 000 mt of dogfish were discarded, of which 1 4 000 mt were killed. LOWESS-smoothed minimum swept-area biomass estimates reveal a five-fold increase in abundance over the last 30 years to about 65 0 000 mt in 1993. There is no evidence, however, of a continuing increase i n the fishable stock (i.e., greater than or equal to 80 cm in length) since 1990. Mean lengths of dogfish in commercial landings and research survey c atches have decreased in the last five years, and fraction of females great er than or equal to 80 cm in the survey has declined markedly. Change-in-se x-ratio estimators were used to derive sex-specific F values. Results sugge st a five-fold increase in F on females from 1991 to 1993 to levels in exce ss of 0.25 per year. Commercial fishery information, research survey data, and life history para meters from the literature were used to develop a size- and sex-structured equilibrium model for dogfish. Using the concept of pups per recruit, preli minary 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 long evity of this species and experiences in shark fisheries worldwide, plans t o increase exploitation rates should proceed cautiously to avoid rapid depl etion of the adult stock. (C) 1998 Elsevier Science B.V. All rights reserve d.