Epsilon Archive for Student Projects

Abstract

Forage fish occupy a critical intermediate trophic position in marine ecosystems and they represent a key resource for seabird breeding success. Cury et al. (2011) illustrated consistently declining seabird breeding productivity below abundances of one third of maximum historical fish stock biomass (B1/3), suggesting a quantitative conservation threshold. Commercial fisheries management, however, is predominantly guided by maximum sustainable yield (MSY) targets, and the compatibility of these targets with the B1/3 threshold remains poorly examined at a fish stock scale. This study assessed the sustainability of spawning stock biomass management targets (SSBmsy RPs) relative to Cury's threshold (B1/3 RP) for 70 globally distributed forage fish stocks from the RAM Legacy Stock Assessment Database. A Seabird Sustainability Index (SSI) was introduced as the ratio of SSBmsy RP to B1/3 RP. Additionally, stock-wide biomass densities were derived using the GRSF Areas Database. The RP relationship, the effect of time series length, density translation adequacy, and the influence of qualitative stock variables were examined. The median SSI of 1.131 indicates that SSBmsy targets are on average sufficient to sustain reproductive success, yet 39% of stocks displayed an SSI < 1. However, only 46% of stocks recently exceeded their SSBmsy target, and 41% had recent SSB estimates below the B1/3 RP, indicating prey abundances associated with reduced seabird breeding success. A potential influence of time series length on the SSI was observed, consistent with the shifting baseline syndrome, though explanatory power remained weak. Density medians ranged from 0.331 to 0.493 MT/km² and revealed a strong negative relationship with stock area size. Qualitative variables showed limited explanatory power, with observed patterns more reflective of management structures than ecological drivers. These findings highlight that current management targets are only partially aligned with seabird conservation needs and that consistent implementation of management targets would represent a meaningful improvement for both fisheries sustainability and seabird conservation. The B1/3 RP is proposed as an additional conservation threshold within a broader ecosystem-based management framework, while limitations of single-species management and data constraints are discussed.