Habitat compression and ecosystem shifts as potential links between marine heatwave and record whale entanglements

Climate change is increasing the frequency, duration, and intensity of marine heatwaves (MHWs), posing new challenges for fisheries management and conservation of protected species. One significant impact is the rise in whale entanglements in fishing gear, often exceeding the rate of population recovery. The California Current Large Marine Ecosystem (CCLME), a productive upwelling system, experienced a significant MHW from 2014–2016. This MHW caused a sustained bloom of toxic *Pseudo-nitzschia* diatoms, resulting in domoic acid contamination and an unprecedented delay in the opening of the California Dungeness crab fishery. This delay, coupled with the MHW's impact on the ecosystem, coincided with a dramatic increase in whale entanglements, primarily involving humpback whales. This study uses an ecosystem perspective to examine the relationship between oceanographic conditions during the MHW, changes in forage species distribution and availability, and the increased whale entanglements. The central hypothesis is that the MHW-induced upwelling habitat compression caused changes in forage availability, leading to a shift in whale distribution towards the nearshore and an increased overlap with crab fishing gear. The delayed crab fishery opening further exacerbated this overlap.

The introduction cites numerous studies highlighting the impacts of climate change, MHWs, and their effects on marine life and fisheries. It mentions previous research on whale entanglements globally, emphasizing the need to understand climate extremes' impact on whale habitats and fisheries. Existing literature on the CCLME's upwelling dynamics, nutrient supply, and forage species distribution is also reviewed. The authors reference studies showing the wide-ranging impacts of the 2014–2016 MHW, including the toxic algal bloom and its economic consequences for fisheries. Studies on humpback whale foraging behavior, diet, and population recovery are also reviewed to establish the baseline understanding relevant to the research.

The study integrates data from multiple sources to assess the hypothesis. A Habitat Compression Index (HCI) was developed using data from a data-assimilative oceanographic model to monitor changes in the areal extent of cool sea surface temperature (SST) habitat. This HCI allowed for a long-term perspective on periods of enhanced or decreased upwelling habitat. The analysis focused on March and May due to the seasonal progression of upwelling winds. Confirmed whale entanglement records from NOAA Fisheries were analyzed to examine temporal trends. Fishery landings data from the California Dungeness crab fishery provided information on fishing activity. Data from an ecosystem assessment survey monitoring mid-water forage species distribution and biodiversity (Rockfish Recruitment and Ecosystem Assessment Survey, RREAS) were used to examine changes in krill and anchovy abundance and distribution. The RREAS employed both mid-water trawls and acoustic methods to assess forage species. Spatial intensity of forage species was analyzed using Moran's I. Visual survey data from the RREAS were used to analyze humpback whale abundance and distribution on and off the continental shelf.

The HCI revealed that upwelling habitat was significantly compressed during the MHW, particularly in 2015 and 2016. Epipelagic species richness increased during the MHW, indicating unusual biodiversity. Krill abundance was anomalously low in 2015, while YOY anchovy abundance increased significantly in 2015–2016. Humpback whale relative abundance showed a statistically significant onshore shift from 2014 to 2015–2016. Confirmed whale entanglements, mostly humpback whales, spiked during the MHW, with most entanglements linked to Dungeness crab fishing gear. The delayed opening of the crab fishery in 2016, due to domoic acid contamination, coincided with the peak arrival of humpback whales, increasing the spatial overlap between whales and fishing gear. The analysis of forage species distributions showed shifts in krill and anchovy abundance and spatial intensity patterns throughout the period, potentially driving the observed changes in whale distribution. The combination of habitat compression, forage shifts, and the delayed fishery opening strongly suggests a causal link between the MHW and the increased entanglements.

The findings strongly support the hypothesis that MHW-induced upwelling habitat compression intensified the spatial overlap between whales and crab fishing gear. Changes in forage availability, particularly the decline in krill and increase in nearshore anchovy, likely caused the observed shift in whale distribution. The delayed opening of the crab fishery due to domoic acid contamination further exacerbated the situation. The study highlights the complex interplay between climate change, ecosystem shifts, and human activities in driving entanglement events. The findings underscore the need for ecosystem-based management approaches that consider the impacts of climate change on both fisheries and protected species.

This study demonstrates a clear link between the 2014–2016 MHW, ecosystem shifts, and a dramatic increase in whale entanglements. The findings emphasize the importance of considering climate change impacts on ecosystem dynamics when managing fisheries and protecting marine mammals. The establishment of the California Dungeness Crab Whale Entanglement Working Group and the Risk Assessment and Mitigation Program (RAMP) represent important steps towards mitigating future entanglement risks. Future research should focus on refining risk assessment models that incorporate various ecosystem indicators and dynamic ocean management tools.

The study relies on observational data, and while correlations are strong, it cannot definitively prove causality between the MHW and whale entanglements. The entanglement data are based on reports, which may not capture all entanglement events. The RREAS data provides a strong basis for understanding the changes in forage fish populations; however, there may be limitations in the ability to predict future patterns exactly, particularly in the context of climate change, whose impacts may drive unexpected changes in the system.