Whale entanglements disentangled through the lens of cumulative habitat compression

Abstract

Ten years following a marine heatwave event, whale entanglements continue to challenge ecosystem-based fishery and protected species management in the California Current Large Marine Ecosystem (CCLME). Although new fishing regulations combined with environmental changes resulted in a decline in humpback whale (Megaptera novaeangliae) entanglements from the peak during a prolonged heatwave, reports continue at a higher rate than prior to 2014. The entanglement record is imperfect because many entanglements go unobserved and those that are observed may not correspond to where the entanglement occurred. Thus it remains a challenge to understand when and where entanglements are most likely to occur and under what environmental conditions. Monitoring spatial changes in compression of cool thermal habitat and forage species availability previously permitted detection of ecosystem conditions that result in shoreward shifts in whales that overlapped with fixed gear fisheries. Following the heatwave (2015–2016), a surge in anchovy populations and increased variability of krill abundance, has led to increased concentrations of prey nearshore, fueling whale populations as they continue to recover from historical whaling. We reapply the habitat compression index to examine the association among annual humpback whale entanglement reports, population abundance and cumulative changes in thermal habitat availability throughout the CCLME. Using a two-step analysis, we document that reported entanglements, alone and adjusted for population growth, are highest during years of low thermal habitat area. We discuss how this relationship can be used to inform fishery management decisions through application of short-term forecasts of habitat compression, and how it can benefit whale entanglement mitigation.