Critical knowledge gaps: estimating potential maximum cumulative anthropogenic mortality limits of key marine mammal species to inform management

Project Number:



SARDI Food Safety and Innovation

Principal Investigator:

Alice Mackay

Project Status:


FRDC Expenditure:



Environment, Industry


Management and mitigation of the bycatch of protected species is required under the EPBC Act and the Fisheries Act. Bycatch trigger limits provide a framework to manage marine mammal bycatch rates and are used by AFMA in the management of the SPF and in the gillnet sector of the Southern and Eastern Scalefish and Shark Fishery (SESSF). The setting of trigger limits requires quantitative information on population size to ensure that the impact of fishing mortality does not negatively affect population status. Robust population estimates do not exist for most marine mammal species in Australian waters and are particularly limited for cetaceans and there is also limited to no information on the distribution and population structure of these species. Bycatch trigger limits need to consider the smallest population unit to ensure that levels of anthropogenic mortality are sustainable. Where data are sparse, it can be difficult to reach consensus between different stakeholder groups on the validity of management measures, particularly in relation to trigger limits. By eliciting expert knowledge through a formally structured system, a transparent process of evaluating and synthesising current data and quantifying the uncertainty around proposed bycatch trigger limits is available for managers and can be used to build industry and stakeholder support. This is particularly important when considering the management of bycatch impacts on populations that interact with a number of different jurisdictions.


1. Collate and synthesise all available data on the distribution, abundance and population structure of key marine mammal species that overlap with the area of the SPF.

2. Convene an Expert workshop to “review current information available to inform the establishment of trigger limits for key marine mammal species (especially the short-beaked common dolphin, Australian fur seals and long-nosed fur seal).”

3. Report on the outcomes of this workshop and present the results of PBR analysis for short-beaked common dolphins and seals , based on available data, expert opinion and a precautionary approach.

4. Identify knowledge gaps and research needs to improve quantitative robustness of PBR of each species.

Final Report - 2015-035 - Critical knowledge gaps: estimating potential maximum cumulative anthropogenic mortality limits of key marine mammal species to inform management

Final Report
Date Published:September 2016

Principle investigator: Alice Mackay, Simon Goldsworthy, and Peter Harrison - South Australian Research and Development Institute (Aquatic Sciences)​

Key words: Marine mammal, Small Pelagic Fishery, population estimate, elicitation review, Australian fur seal, bottlenose dolphin, Indian Ocean bottlenose dolphin, Fraser’s dolphin, hourglass dolphin, Risso’s dolphin, southern right whale dolphin, striped dolphin, Potential Biological Removal .

The Commonwealth Small Pelagic Fishery (SPF) has attracted significant public attention as a result of marine mammal bycatch (common dolphins and fur seals) in mid-water trawl operations by the FV Geelong Star since it commenced fishing in April 2015. The Australian Fisheries Management Authority (AFMA) currently has a number of management measures in place to minimise further interactions between marine mammals and the fishery. These include spatial and temporal closures based on bycatch trigger limits.

A two day workshop funded by the Fisheries Research and Development Corporation (FRDC) held in June 2015 explored further mitigation options to reduce interaction rates with marine mammals. An outcome of this meeting recommended that an expert group be established to review available information used to establish trigger limits for key marine mammal species in the area of the SPF.

The FRDC established this project to follow up on recommendations and achieve the following:

  • Collate, synthesise and review all available data on the distribution, abundance and population structure of key marine mammal species that overlap with the area of the SPF
  • To inform management of interactions with key marine mammal species and use structured expert elicitation to assess plausible minimum population sizes of these species in the area of the SPF. Marine mammal species considered were Australian sea lions (ASL), long-nosed fur seals (LNFS), Australian fur seals (AUFS), short-beaked common dolphins (SBCD) and bottlenose dolphins. Report on the outcomes of the expert workshop and use these outputs where possible to calculate Potential Biological Removal where possible post-workshop for short-beaked common dolphins and seals.

Potential Biological Removal calculates the maximum number of anthropogenic mortalities a population can sustain while still allowing that population to reach or maintain its optimum sustainable population. In other words, the limit that the population can bear before there are adverse impacts on the population. The calculation of PBR is relatively simple but requires data on the abundance of a given population and an understanding of the range or spatial distribution of that population.

  • Identify knowledge gaps and research needs to improve quantitative robustness of bycatch limit reference points for each species.

A two day workshop was convened at SARDI Aquatic Sciences, Adelaide on 19-20 October 2015: The first day of the workshop provided stakeholders from Commonwealth and State fisheries, environment agencies, industry and environmental organisations with a summary of currently available information on the abundance and distribution of key marine mammal species in the area of the SPF.

On the second day, a Closed Technical Workshop (CTW) of invited scientists was held. During the Closed Technical Workshop a process of expert elicitation was used to elicit estimates of population size for each marine mammal species relative to different spatial zones. Estimates were based on available data on marine mammal abundance and were not provided for those species or zones where there was no data.

Estimates obtained through CTW and Expert Elicitation process were used, where possible, to calculate Potential Biological Removal (PBR) for the species considered. The outputs of the Expert Elicitation were used where possible to calculate PBR for species and management zones. While recent abundance data were available for the seal species considered, available data on dolphin population structure and size were extremely limited. 

Key Project Results:

  • The technical workshop produced a synthesis of available data and highlighted key data gaps with respect to abundance, distribution and population structure of marine mammal species considered. The participants agreed that expert elicitation was not an appropriate method for estimating population abundance where empirical data was lacking.
  • The project identified that in order to use methods such as PBR to calculate limits to anthropogenic mortality to a given population, the key data required is a recent robust estimate of abundance and the spatial extent of that population.
  • In general there is good data available on the distribution, abundance and population structure of pinniped species (Australian sea lions, long-nosed fur seals and Australian fur seals).  However, such data are lacking for dolphin species for much of the area considered.
  • For Australian sea lion PBR was calculated at the colony level based on the high degree of population structuring of the species across its range. PBR estimates ranged from 0 to 44 individuals.
  • PBR calculated for long-nosed fur seal ranged from 81 to 4,498 depending on the spatial management zone considered. For Australian fur seals estimated PBR for a single management zone ranged 2,623 to 4,721.
  • A PBR of 261 individuals was calculated for short-beaked common dolphins in management zone 3. This PBR was based on an abundance estimate obtained in a small part of the zone. PBR assumes that the abundance estimate is for the total population, this may not be the case, given the small area surveyed and recent studies of population sub-structure in the region.
  • Three key data requirements are needed in order to manage potential impacts of anthropogenic removals on marine mammal populations. These are robust estimates of population abundance and range. An understanding of population structuring in a species and estimates of cumulative removal levels from all anthropogenic sources, including fisheries.