More analytical bite in estimating targets for shark harvest

Abstract

The typically slow life histories of sharks make them susceptible to overexploitation. However, this characteristic also means that shark populations are more amenable to markrecapture estimation of vital rates and to population viability analysis, compared to many teleost (bony) fishes. We applied this novel approach to inform fisheries management for 2 shark species (Carcharhinus tilstoni and C. sorrah) in northern Australia. We calculated survival estimates based on a current mark-recapture study and age-frequency distributions from historical catch data, and used these along with other estimates of vital rates to construct an individual-based population viability analysis (using VORTEX software). We estimated total current mortality (natural and fishing-related) from tagging data using Brownie models (mean ± SE: 0.532 ± 0.097 and 0.487 ± 0.136) and recapture rate (fisheries-related mortality) (0.023 ± 0.005 and 0.008 ± 0.003) for C. tilstoni and C. sorrah, respectively. Based on historical age-at-length and age-frequency data, mean survival across ages after historical overexploitation was similar for both species (∼0.43, or ∼0.33 as an age frequency-weighted mean). After correcting for capture-related mortality and permanent emigration, VORTEX population projections including incrementing simulated harvest suggest that the current rates of harvest of both species result in population trajectories that are approximately stable (20 yr predicted average rate of population change r ≈ 0), but this is complicated by a variable contribution of the slower-reproducing species C. limbatus within the black-tip shark complex (C. limbatus is commonly misidentified as C. tilstoni in the fishery). VORTEX projections also clearly indicated that the fishery was overexploited in the past. Using multiple datasets and an individual-based modelling framework to estimate harvest limits for a commercial fishery provides novel insights into management. This is especially pertinent to shark populations given the relative tractability of marking and recovering individuals and the inherent suscepti bility of shark populations to overexploitation.