Conservation management in the face of uncertainty: effectiveness of four options for managing Hector’s dolphin bycatch

Abstract

New Zealand's endemic Hector's dolphins Cephalorhynchus hectori overlap with gillnet and trawl fisheries throughout their geographic range. The catch rate in commercial gillnets has been estimated using independent observers. No quantitative estimates are available for bycatch in recreational gillnets or trawl fisheries. The catch rate (per dolphin, per km of gillnet, per year) in commercial gillnets was used in a population viability analysis to estimate past and future population sizes. Total population size today (7873, coefficient of variation [CV] 0.16) was estimated at 27% of population size in 1970 (29316, CV 0.16), before a major expansion of commercial gillnetting. The species matches IUCN criteria for 'endangered'. The North Island subspecies matches criteria for 'critically endangered'. Current management, which includes 2 protected areas, is not sufficient to halt population declines. Hector's dolphin populations are predicted to continue declining to 5475 (CV 0.20) by 2050. Creating 4 strategically placed protected areas would allow population recovery towards 1970 levels, with an estimated 47% probability of reaching 50% of 1970 population size by 2050 (proportion of 5000 model runs in which final population size was at or above this reference level). Reducing fisheries mortality to levels approaching zero shows the strongest promise of meeting national and international guidelines for managing dolphin bycatch, with a 59% probability of reaching 50% of 1970 population size by 2050.

Sea otter mortality in fish and shellfish traps: estimating potential impacts and exploring possible solutions

“…As with other studies that have had to rely on indirect or incomplete information to evaluate bycatch impacts (Arnold et al 2006, Slooten 2007, Veran et al 2007, Moore & Read 2008), our results demonstrate that the lack of direct estimates of bycatch rates does not justify ignoring a potential problem that could thwart the recovery of a threatened species. …”

“…Even localized small-scale fisheries can result in unsustainable levels of bycatch (Peckham et al 2007, Campbell et al 2008). However, in many cases there is no reliable method to quantify bycatch, to estimate its effect on the demography of affected species (Poole et al 2007, Slooten 2007, Moore & Read 2008, Zydelis et al 2009), or even to determine when and where it is occurring (Veran et al 2007). In such cases, it therefore may be necessary to rely on indirect evidence and simulation analyses to estimate population-level impacts of bycatch (Arnold et al 2006, Slooten 2007, Veran et al 2007, Moore & Read 2008.…”

Criticism is unfounded: reply to Middleton et al. (2007)

“…argue that some of the modelling choices in my analysis of Hector's dolphin Cephalorhynchus hectori bycatch (Slooten 2007) lead to an overestimation of the impact of fishing on this species and of the potential for population recovery if fishing mortality were reduced to zero. This reply explains the rationale behind the model choices in more detail and shows that they do not influence the results in the way that Middleton et al (2007) claim.…”

Hector's dolphin risk assessments: old and new analyses show consistent results

“…Fisheries modelling approaches have also been used, with a deterministic Schaefer model (Martien et al 1999), followed by stochastic models (Burkhart & Slooten 2003). The stochastic Schaefer model was used to assess the effectiveness of different management options for Hector's dolphin bycatch in fisheries (Slooten 2007;Slooten & Dawson 2010). A third analysis approach used a Bayesian model (Davies et al 2008), originally constructed for New Zealand sea lion (Breen et al 2003).…”