Abundance and Potential Biological Removal of Common Dolphins Subject to Fishery Impacts in South Australian Waters

Abstract

Conservation management of wildlife species should be underpinned by knowledge of their distribution and abundance, as well as impacts of human activities on their populations and habitats. Common dolphins (Delphinus delphis) are subject to incidental capture in a range of Australia’s commercial fisheries including gill netting, purse seining and mid-water trawling. The impact these fishery interactions have on common dolphin populations is uncertain, as estimates of abundance are lacking, particularly for the segments of the populations at risk of bycatch and in greater need of protection. Here we used double-observer platform aerial surveys and mark-recapture distance sampling methods to estimate the abundance of common dolphins in 2011 over an area of 42,438 km2 in central South Australia, where incidental mortality of common dolphins due to fisheries bycatch is the highest. We also used the potential biological removal (PBR) method to estimate sustainable levels of human-caused mortality for this segment of the population. The estimated abundance of common dolphins was 21,733 (CV = 0.25; 95% CI = 13,809–34,203) in austral summer/autumn and 26,504 in winter/spring (CV = 0.19; 95% CI = 19,488–36,046). Annual PBR estimates, assuming a conservative maximum population growth rate of Rmax = 0.02 and a recovery factor of Fr = 0.5 for species of unknown conservation status, ranged from 189 (summer/autumn) to 239 dolphins (winter/spring), and from 378 (summer/autumn) to 478 dolphins (winter/spring) with an Rmax = 0.04. Our results indicate that common dolphins are an abundant dolphin species in waters over the central South Australian continental shelf (up to 100 m deep). Based on the 2011 abundance estimates of this species, the highest estimated bycatch of common dolphins (423 mortalities in 2004/05) in the southern Australian region exceeded the precautionary PBR estimates for this population segment. Recent bycatch levels appear to be below PBR estimates, but low observer coverage and underreporting of dolphin mortalities by fishers means that estimates of dolphin bycatch rates are not robust. The effects of cumulative human impacts on common dolphins are not well understood, and thus we recommend a precautionary management approach to manage common dolphin bycatch based on local abundance estimates.

Drone images afford more detections of marine wildlife than real-time observers during simultaneous large-scale surveys

“…Although the probability of individual observers can vary significantly ( e.g. , in Pollock et al (2006) probabilities ranged from 0.24 to 0.9) the overall probability of a tandem team seeing a dugong is generally quite high (in the Pollock et al example, probabilities ranged from 0.87 to 0.96, and similarly, Parra et al (2021) and Slooten, Dawson & Rayment (2004) report detection rates of small dolphins by double-observer teams as 0.91–0.99 and 0.96 respectively. Both sets of observers from our observer survey team were estimated to have seen 95% of dugongs available to be detected.…”

Quantifying Abundance and Mapping Distribution of Loggerhead Turtles in the Mediterranean Sea Using Aerial Surveys: Implications for Conservation

“…While, to ease comparisons, further mathematical exercises are possible to extrapolate abundance estimates and PBRs to areas not directly surveyed, this approach is discouraged. Abundance estimates and thus PBR values extrapolated to nonsurveyed areas can be strongly biased due to the fact that the correlates between species densities and covariates in extrapolated areas are unknown [94,95]. It is also important to stress that bycatch rates are often biased (i.e., under-reported [13]) and this adds further difficulties in understanding the actual contribution of commercial fisheries to loggerhead turtle mortality in support of management and conservation.…”

A negative trend in abundance and an exceeded mortality limit call for conservation action for the Vulnerable Belt Sea harbour porpoise population

“…In the case of the MMPA, meeting the conservation objective is defined on two criteria; 1) that a population at the maximum net productivity level [MNPL, greater than or equal to 50% of carrying capacity (K)] is able to remain there for 20 years (short-term goal), and 2) that a population that is depleted at 30% of K is able, despite non-nil removals, to reach and remain above MNPL within 100 years with a probability of 0.95 (long-term goal; see 'conservation goals' in Wade, 1998, page 8). The PBR control is enshrined in the U.S. MMPA but has been applied to a range of cetacean species around the world (Slooten et al, 2006;Stenson et al, 2012;Parra et al, 2021) thereby accepting, either implicitly or explicitly, the U.S. MMPA conservation objective.…”