SlotLim: Catch advice for data‐limited vertebrate and invertebrate fisheries managed by harvest slot limits

Abstract Minimum landing sizes are a fisheries management tool conventionally used to exclude juveniles from fisheries. Harvest slot limits (HSL) are an alternative fisheries management tool used to exclude both juveniles and larger individuals from a fishery through the implementation of both minimum and maximum landing sizes. However, fishery‐dependent data from HSL‐managed fisheries are only representative of a small portion of the population. These data do not meet the requirements for conventional stock assessments nor harvest control rules, so these fisheries cannot be assessed without additional and expensive fishery‐independent data. The objective of this research was to develop a novel harvest control rule able to produce catch advice for fisheries managed by HSL using fishery‐dependent data. The SlotLim method, and corresponding R package, were developed and applied to the data‐limited Scottish live ballan wrasse Labrus bergylta fishery. Within SlotLim, the advised catch is a product of the previous catch and two multipliers: the targeted biomass adjustment (TBA) and size adherence multiplier (SAM). The TBA assesses the rate of change in an abundance or biomass index, adjusted for the proportion of the population targeted by HSL. The SAM assesses fishers' compliance with HSL. The methodology follows a simple premise: the advised catch increases with increasing abundance/biomass indices and adherence to HSL. The minimum data requirements are two consecutive years of catch, length frequency and an abundance or biomass index (all from fishery‐dependent sources), species‐specific growth rate coefficients and the natural mortality rate. The SlotLim method advised catch for a reduction in catch by 17% for the Scottish ballan wrasse fishery due to an observed reduction in abundances indices and non‐adherence to maximum landing sizes. Solution. The SlotLim method allows HSL‐managed fisheries to be assessed at limited expense, contributing to the continued sustainable use of these resources. HSL may also be considered a viable strategy for other data‐limited fisheries upon the availability of this harvest control rule.