Optimal harvest regulations under conflicting tradeoffs between conservation and recreational fishery objectives

Abstract

Length-based harvest regulations alter the fishing-induced demographic and evolutionary trajectories of exploited stocks and thus shape the existing tradeoffs among fishery and conservation objectives. We used a structurally realistic eco-genetic individual-based harvest model that implements dynamic angling mortality and cryptic mortality sources (illegal harvest and hooking mortality). We (1) analyzed the effects of alternative length-based harvest regulations under scenarios involving different combinations of exploitation intensity and hooking mortality on a suite of indicators of fishery performance and conservation status of a freshwater fish stock, and (2) determined the regulations that optimize the tradeoff among selected indicators under different management strategies, and fishery and conservation objectives. Fishing scenarios under a maximum-length limit regulation maximized harvest yield but led always to recruitment overfishing, irrespective of the exploitation and hooking mortality rates simulated. Fishing scenarios under a harvest slot limits regulation (HS) were best at maintaining a high status of old, large, fecund fish and a more natural age-structure with higher biomass and reproductive potential, performing increasingly better than minimum-length limit (MLL) regulations with decreasing hooking mortality. Both regulation types were effective at preventing overexploitation and only under scenarios with low restrictiveness and high exploitation intensity and hooking mortality was the stock at risk of recruitment overfishing. MLLs outperformed HS regulations in terms of fishery performance, consistently presenting greater harvest yield and efficiency, and size of harvested fish. High rates of hooking mortality rendered HS regulations less effective than assumed, so they were always outperformed by MLLs irrespective of the management strategy and objectives. When hooking mortality was low, HSs constituted the optimal regulation type in most cases except when high fishery performance was favoured over conservation objectives or harvest of large fish was regarded as critically important.