An age-structured model showing the bene®ts of marine reserves in controlling overexploitation Sylvie Gue Ânette * , Tony J. Pitcher Fisheries Centre, University of British Columbia, 2204 Main Mall, Vancouver, BC, Canada V6T 1Z4 Received 5 August 1997; accepted 3 August 1998 Abstract Previous modelling of areas closed to ®shing (marine reserves) has generally employed non-dynamic models and has not included biological factors such as stock±recruitment and weight±fecundity relations. These models predicted that a marine reserve would result in a decrease in ®shery yield, an increase in spawning biomass and that movements of ®sh across the reserve boundaries could reduce its bene®ts. We utilised an age-structured model based on an Atlantic cod population that included more realistic reproductive factors. We compared a Reserve regime that contained a reserve with a No-reserve regime in which the usual ®shery management tools were used. As exploitation rate increased, the relative recruitment and spawners biomass decreased in the No-reserve regime. Larger reserves resulted in more robust recruitment and biomass of spawners. At low exploitation rates, marine reserves resulted in smaller yields. However, when the exploitation rate was larger than the rate which gives the maximum sustainable yield, the biomass of female spawners was maintained at a higher level in the Reserve regime and hence the yield did not collapse. Faster rate of movement of ®sh decreased these advantages, but the higher spawners biomass and level of recruitment still provided advantages for the Reserve regime. Moreover, even for highly mobile ®sh, our model suggests that a ®sh stock protected with a marine reserve would be more resilient to exploitation than when managed without. However, a model realistic spatially and temporally would be necessary to assess the usefulness of marine reserves to prevent overexploitation of migrating ®sh. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Dynamic pool model; Marine reserves; Atlantic cod; Management; Spawner biomass 1. Introduction In the face of continuously high pressure on ®sh stocks, marine reserves (areas closed to ®shing) are emerging as an effective tool to protect populations from exploitation by putting a part of a population out of the reach of ®shers (Polacheck, 1990; Dugan and Davis, 1993; Rowley, 1994; Bohnsack, 1996; Lauck et al., 1998). Beverton and Holt (1957) ®rst considered the usefulness of closed areas for plaice in the North Sea using yield-per-recruit modelling but despite a potential increase in yield, they advised against the use of marine reserves because such a restriction on ®shers was likely to increase the cost of ®nding ®sh. Also the importance of detailed analysis of spatial dispersion of ®sh was thought to be essential to assess the usefulness of marine reserves. Therefore Beverton and Holt recommended other regulatory measures for ®sheries such as ¯eet reduction, catch reduction and minimum Fisheries Research 39 (1999) 295±303 *Corresponding author. Tel.: +1-604-822-2731; fax: +1-604- 822-8934; e-mail: sylvie@fisheries.com 0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00173-8