Implications of life history uncertainty when evaluating status in the Northwest Atlantic population of white shark (Carcharodon carcharias)

Abstract

To effectively protect at‐risk sharks, resource managers and conservation practitioners must have a good understanding of how fisheries removals contribute to changes in abundance and how regulatory restrictions may impact a population trajectory. This means they need to know the number of animals being removed from a population and whether a given number of removals will lead to population increases or declines. For white shark (Carcharodon carcharias), theoretical quantities like the intrinsic rate of population increase or rebound potential (ability to increase in size following decline) are difficult to conceptualize in terms of real‐world abundance changes, which limits our ability to answer practical management questions. To address this shortfall, we designed a simulation model to evaluate how our understanding of longevity and life history variability of white shark affects our understanding of population trends in the Northwest Atlantic. Then, we quantified the magnitude of removals that could have caused historical population declines, compared these to biologically based reference points, and explored the removal scenarios which would result in population increase. Our results suggest that removals on the order of 100s of juveniles per year could have resulted in population‐level declines in excess of 60% during the 1970s and 1980s. Conservation actions implemented since the 1990s would have needed to be nearly 100% effective at preventing fishing mortality in order for the population to double in abundance over the last 30 years. Total removals from all fleets needed to be exceptionally small to keep them below biological reference points for white shark in the Northwest Atlantic. The population's inherent vulnerability to fishing pressure reaffirms the need for restrictive national and international conservation measures, even under a situation of abundance increase.

Effective number of white shark (Carcharodon carcharias, Linnaeus) breeders is stable over four successive years in the population adjacent to eastern Australia and New Zealand

“…Catches of white shark and other shark species are only recently increasing year‐on‐year (Department of Primary Industries, 2019) following long term declines over 80 years of the bather‐protection program along the east coast of Australia, which has been lethal for sharks despite catch‐and‐release programs (Roff et al., 2018). The recent modeling of the recovery of the North West Atlantic white shark population provides a useful principal in this regard; “every fish counts” (Bowlby & Gibson, 2020, p.9).…”

Beyond Post-release Mortality: Inferences on Recovery Periods and Natural Mortality From Electronic Tagging Data for Discarded Lamnid Sharks

“…In all, there was fair correspondence with rates derived from life history for both species, even though our data came primarily from juvenile animals. Estimates of M based on age and growth parameters, maturity, and longevity most often yield a single value, and variability is generated by applying different types of estimators (e.g., Cortés, 2002Cortés, , 2016 or by allowing for variability in longevity when using a single estimator (e.g., Bowlby and Gibson, 2020). Using the Then et al (2015) suite of estimators based on longevity and growth data, M ranged from 0.081 to 0.267 for porbeagle and from 0.068 to 0.318 for shortfin mako for males and females combined.…”

How continuing mortality affects recovery potential for prohibited sharks: The case of white sharks in South Africa

“…We adapted the simulation model described in Bowlby and Gibson (2020) to characterize productivity and predict how varying levels of incidental mortality may influence abundance trends. The first component of this model is a life table analysis describing population productivity, which was used to determine the intrinsic rate of population growth (r) from demographic information (Mollet and Cailliet, 2002;Corteś, 2016).…”