Predicting the effects of whale population recovery on Northeast Pacific food webs and fisheries: an ecosystem modelling approach

Abstract

The recovery of whale populations from historical depletion may have the potential to noticeably affect Northeast Pacific ecosystems and fisheries. Surplus production models based on whaling catch records were used to reconstruct the historical abundances of five large whale species in the waters surrounding Haida Gwaii, British Columbia. The results suggest that the local abundances of all five species were vastly higher before the onset of modern whaling. A comparison of ecosystem models representing the states of the local marine food web before and after full whale recovery indicates that abundant whales could consume large proportions of the annual production of their principal prey, ranging up to 87% for Pacific herring (Clupea pallasii) and 72% for piscivorous rockfish (Sebastes spp.). Dynamic modelling of the food web effects of whale recovery, including simulations of simultaneous top‐down and bottom‐up forcing and a Monte Carlo sensitivity analysis, revealed noticeable (∼6–12%) top‐down effects on Pacific herring biomass owing to increased predation by humpback and fin whales. However, these effects cannot explain the magnitude of recent declines in local herring biomass. The dynamic modelling results also suggest that top‐down effects of whale recovery could result in reduced biomasses of large rockfish as a result of predation by sperm whales, as well as potential cascading effects on many demersal fish groups. These findings have numerous practical implications for ecosystem‐based fisheries management and whale conservation strategies in Northeast Pacific waters.

Ecosystem effects of invertebrate fisheries

“…Sea otters in the northern BC model were also only 16% of the estimated historical biomass in 1950. Additionally, in the northern BC model, many populations of large whales had been drastically reduced in numbers by 1950 (Surma and Pitcher ), which were not captured in the model, suggesting that whales may be more affected by invertebrate fisheries than represented in our simulations.…”

The highs and lows of herring: A meta‐analysis of patterns and factors in herring collapse and recovery

“…Seasonal and local estimates of their numbers may be available (e.g. Bishop, Watson, Kuletz, & Morgan, 2015; Surma & Pitcher, 2015; Teerlink et al, 2015), but estimates that are relevant to analysis were not generally available. Nevertheless, predator–prey relationships of cetaceans and seabirds on herring have been implicated as top‐down controls on herring (Moran et al, 2018; Read & Brownstein, 2003; Straley et al, 2017) and as recipients of bottom‐up effects from herring (Pikitch et al, 2012, 2014; Smith et al, 2011).…”

“…Extrinsic factors include bottom‐up influences on recruitment and growth originating from the physical environment (Brunel & Dickey‐Collas, 2010; Hay, Rose, Schweigert, & Megrey, 2008; Ito et al, 2015; Williams & Quinn, 2000). Strong top‐down influences from predators have also been suggested as important for herring (Moran, Heintz, Straley, & Vollenweider, 2018; Read & Brownstein, 2003; Schweigert, Boldt, Flostrand, & Cleary, 2010; Surma & Pitcher, 2015; Tjelmeland & Lindstrøm, 2005). Finally, just like questions about whether chicken or the egg came first, there has been a long debate about whether low recruitment is a result of low spawning biomass, or low spawning biomass is a result of periods of low recruitment for herring (Gilbert, 1997; Myers & Barrowman, 1996), with more recent evidence backing recruitment‐driven biomass in forage fish (Szuwalski et al, 2019; Szuwalski, Vert‐Pre, Punt, Branch, & Hilborn, 2015).…”

Ecosystem modelling to quantify the impact of historical whaling on Southern Hemisphere baleen whales

“…For example, the Oceanic subpopulation of humpback whales in the Pacific are still critically endangered (Childerhouse et al., ), despite our model showing fast recovery for this species in that region. Interestingly, our study demonstrates similarities between historical whale trajectories and their recovery in the Southern Hemisphere, and serial depletion of whale species in the Northern Hemisphere, whereby whaling in the Canadian North‐East Pacific led to depletion of blue and humpback whales first, and projected humpback recovery has been much faster than for blue or fin whales (Surma & Pitcher, ), largely due to the shorter breeding cycles of humpback whales.…”