Harvest control rules for the Western horse mackerel (Trachurus trachurus) stock given paucity of fishery-independent data

Abstract

The Western horse mackerel stock, widely distributed in EU waters, is characterized by spasmodic recruitment. Currently, the strength of a year class cannot be confirmed before it is fully recruited to the fishery and has reached the age of 5 years. The only fishery-independent information available is an estimate of egg abundance made every third year. The state of the stock is considered uncertain, and there is no agreed management plan. Following EU requests, a set of harvest control rules (HCRs) that allows for the increased proportion of juvenile fish taken by the fishery in recent years was tested by simulation. The proposed HCRs are based either on the results of a full assessment (Constant-proportion strategy) or simply on the egg estimate used as an indicator of stock status (Slope strategy). Biological risk is compared for scenarios where: (i) uncertainty regarding stock dynamics and in the relationship between egg data and spawning-stock biomass is high (current situation); (ii) variable fractions of the total allowable catch (TAC) are taken in the areas occupied by juveniles and adults; and (iii) there is an implementation error. Results suggest that taking a larger portion of the TAC in the area occupied by juveniles increases the risk of impaired recruitment. Comparison of the Constant-proportion and Slope strategies suggests that the former is more conservative, if the assessment is unbiased or if bias is low. Given the paucity of fishery-independent data, a strategy resulting in an approximately constant catch may be appropriate for this stock. © 2007 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved.