Reproductive contributions foregone with harvesting: a conceptual framework J.F. Caddy a,b,* , J.C. Seijo a,c,1 a Centro de Investigacio ´n de Estudios Avanzados del IPN, Unidad Me ´rida, Apto. Postal 73, Me ´rida 97300, Yucata ´n, Mexico b Dept. of Environmental Science and Technology, University of London, London, UK c Centro Marista de Estudios Superiores, Perife ´rico Norte Tablaje 13941 Carretera Me ´rida-Progreso, Me ´rida 97300, Yucata ´n, Mexico Received 15 February 2001; received in revised form 5 December 2001; accepted 5 January 2002 Abstract Undiscounted changes in the value of female fish from birth to maturity are calculated at equilibrium given a vector of natural mortality at age. Two types of valuations are compared; the summed harvested value of cohorts under given natural and fishing mortality vectors, and the reproductive contribution of different ages lost on capture. Incorporating a fecundity-at-age relationship allows comparison of reproductive and harvest values at age assuming inter-generational population stability. Sample calculations for Mediterranean hake (Merluccius merluccius) with parameter values from the literature suggest that the reproductive value of large females considerably exceeds their landed value after ages 4–5, and that the age at maximum reproductive value of individuals already in the population increases with exploitation rate. The implications of this type of calculation for management of heavily exploited Mediterranean hake stocks is that the focus of inshore trawl fishery should be mainly on older juvenile fish. Since the population is largely supported by the reproductive contribution of older mature hake offshore, conservation measures in the absence of a significant reduction in fishing effort should explicitly protect older spawners through use of the refugium concept and/or by more selective fishing gear. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Market and reproductive values; Reference point; Survival probability matrix; Parental age; Hake; Fisheries for juveniles 1. Introduction 1.1. Systems of valuation of fish in the sea In the harvest of wild marine resources an economic value is usually assigned only at the moment of capture of a resource which an instant previously was common property. This applies even to quota systems such as ITQ management, where access is limited to those exercising ‘stewardship’ (as opposed to ownership) over the uncaught resource. Gavaris (1996) referred to the need to extend ITQ systems to incorporate the value of uncaught fish and reduce the incentive to catch the entire quota under unfavour- able market conditions. He noted that ‘‘age-based accounting allows us to properly measure the natural capital represented by uncaught fish’’. Assigning a value to fish in the sea would be an essential feature of systems such as bankable quotas (Townsend, 1992) which would allow fishers to carry over uncaught Fisheries Research 59 (2002) 17–30 * Corresponding author. Present address: Via Cervialto 3, 04011 Aprilia, Latina, Italy. Tel.: þ39-06-925-6933. E-mail addresses: caddy.j@pcg.it (J.F. Caddy), jseijo@cemaes.marista.edu.mx (J.C. Seijo). 1 Tel.: þ52-999-9410302; fax: þ52-999-9410307. 0165-7836/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0165-7836(02)00011-5