Publication Abstract

Managing over-exploitated fisheries with multi-annual compromise strategies

P. Marchal and J.W. Horwood

Most of the north-eastern Atlantic stocks have been increasingly harvested, since the War, above the biological reference points (Fmax, Fo.l, ...) advocated by scientific advisers. By attempting to achieve the agreed targets from the current position, advisers and managers have been facing two main problems. A first difficulty arises from the impracticability of (a) maximizing gross revenues, stabilizing outcomes for the fishing industry, ensuring the safety for the harvested stocks, on the same time; (b) optimizing both short- and long-term outcomes for the fishing industry. A second difficulty is the lack of predictability, several years ahead, inherent in factors influencing decisions. This study aims to tackle this double issue, by exploring the relative short- and long-term performances of various multi-annual and compromise (or composite) management strategies, in relation to the long-term objective of maximizing gross revenues, for the mixed species multi-metiers fisheries of the Celtic Sea. The management is split intoseveral (D-year "resolution periods". Multi-annual fishing efforts by metier areirreversibly set in advance for a resolution period, at the end of which they are updated. They are calculated to fulfil a %,-weighed compromise amongst three criteria: (i) reaching a "mobile target", which is defined a y -weighed compromisebetween the long-term target and the current level of fishing effort at the beginning of the resolution period; (ii) minimizing variability in fishing effort; (iii) minimizing variability in catches. The parameter y is the key factor for the control of thecomposite strategies. An optimal balance between all the short- and long-term fishery outcomes, as well as the safety for all the stocks, are ensured with a five year resolution period, during which the mobile target is shared between 40-60% of the long-term target and 60-40% of the current fishing effort, while the criteria (i), (ii) and (iii) are equally weighed. The main features of this study are, to a large extent, expandable to most mixed-species multi-metiers fisheries.

Reference:

P. Marchal and J.W. Horwood, 1997. Managing over-exploitated fisheries with multi-annual compromise strategies. Canadian Journal of Fisheries and Aquatic Sciences, 54(10): 2255-2276