It would not be possible to conduct a full stock assessment on all ECTF bycatch species, since over a thousand species are taken. Most of these are taken rarely and there is insufficient knowledge about them and the historical trawl effort applied to estimate the impact of trawling. However, ecologically sustainable management of fisheries resources requires that an ecosystem is managed in its entirety and that bycatch species are not threatened by the fishing activity.
Consequently, a risk assessment needs to be conducted on the vulnerability of bycatch species in the ECTF. Several approaches may be adopted in carrying out such a risk assessment. For example, Western Australia (WA) Fisheries has provided assessment reports on its wild capture fisheries to EA for WTO consideration. WA Fisheries used the component tree analysis developed by the Standing Committee on Fisheries and Aquaculture (SCFA) to detail the risks and management responses for each of the issues identified for the fishery (Chessan and Clayton, 1992).
An approach developed for the NPF (Stobutzki et al., 2001) can be applied to the ECTF. Despite the limitations discussed below, this approach offers a useful method of classifying bycatch information as it becomes available. The first stage in managing bycatch is to identify bycatch species and determine any temporal and spatial variations in distribution. Based on an IUCN system of classification for endangered species, Stobutzki et al. developed a matrix for the classification of bycatch in the NPF. Some 400 species were examined in terms of their “susceptibility to capture” and their “ability to recover” from the effects of trawling. Factors which were considered in determining a species’ susceptibility included position in the water column, preferred habitat, survival, range, day/night catchability, diet and depth range. Factors considered in determining capacity to recover included the probability of breeding, maximum size, removal rate, reproductive strategy, hermaphroditism and mortality index49. An overall score was given to each species according to these two main sets of
criteria and the outcome was presented in the form of a two-dimensional graph (Figure17).
Those species that appeared in the bottom left hand side of the graph were the least sustainable species. They included the Apogonidae (cardinalfish), Arridae, Bathysauridae, Callionymidae (dragonets), Congridae (conger eels), Diodontidae (porcupinefish), Labridae (wrasses), Opsithognathidae (jawfishes), Plotosidae (catfish), Synodontidae (lizardfish) and Tetradontidae (pufferfish). Stobutzki et al. found these species to be highly susceptible to capture by trawl, as they were benthic or demersal, their primary habitat was in soft sediment and their diet included prawns. Species that appeared in the top right hand side of the graph were the most sustainable species. These included the Carangidae (scads and trevallies), Clupeidae (herrings and sardines), Ephippidae (batfish), Scombridae (tunas and mackerels), Sphyraenidae (barracudas) and Terapontidae (trumpeters). These species generally were pelagic, their primary habitat was non-trawl grounds and they had a broad distribution in terms of depth and spatial range.
49 The mortality index relates to the fishing mortality of the population caught. It is determined by how close the
average size of the individuals in the population caught, is to the minimum or maximum size of the species caught in the fishery. The closer the average length of the species caught is to its maximum size in the fishery, the lower the mortality index.
Figure 17: Classification system for quantifying the susceptibility of bycatch species.
Note: The above figure provided courtesy of Dr Stobutzki (former CSIRO)has appeared in several publications, including Stobutzki et al. (2001).
While such a classification is a useful first step in recognising species most at risk, several limitations to this classification approach have been identified. Firstly, the scientific information on the recovery and sustainability of many of the bycatch species is very poor. This forces the assessor to make a judgement on what the “x” or “y” value should be on the graph. Secondly, the approach does not take account of the amount of fishing effort applied to each of these species. As demonstrated by the Far Northern Section study (Poiner et al., 1998), the intensity of trawling has a major impact on the species removed and their recovery. Thirdly, susceptibility to trawl capture is a function of the percentage abundance of the species within trawlable grounds.
Pitcher (CSIRO, pers. comm.) is in the process of developing an approach, which builds on the work of Stobutzki et al.(2001), but also addresses its limitations. As shown in Figure 18, vulnerability indicators for megabenthos are being developed, which take account of species resilience (expressed as percentage survival after trawl) and ability to recover (expressed as recovery rate in the first year after impact).
Quantitative risks can be calculated for these species using information on their estimated vulnerability, the percentage of the population exposed to trawling and the intensity of trawl effort applied. For example, the red species shown in the bottom left hand corner of the graph would be highly vulnerable to trawling if regularly exposed to it.
Such an approach requires some understanding of species biology and fine-scale information on species distribution and abundance and the area of applied trawl effort. To illustrate the point, a species may be highly vulnerable to trawling, but if 98% of its distribution falls outside trawlable grounds, it would be of little management concern. Conversely, a species may be quite robust in withstanding trawl impacts. However, if 98% of this species is found on heavily trawled grounds, it may need to be monitored closely to assess the impact of trawling on its abundance.
Figure 18: Showing the vulnerability indicators for certain megabenthos in the Far Northern Section of the GBR
Marine Park, using a combination of removal and recovery rates.
70 75 80 85 90 95 100 Resilience (% survival/trawl) 0 5 10 15 20 25 30 35 40 45 50 R ec ov er y ra te ( % in 1 st y r) 70 75 80 85 90 95 100 0 5 10 15 20 25 30 35 40 45 50 Turbinaria Cymbastella Semperina
S.suberosa CtenocellaJunceella
Xestospongia Ianthella S.reticulata 70 75 80 85 90 95 100 0 5 10 15 20 25 30 35 40 45 50 Note: 1. The above figure, provided courtesy of Dr Pitcher (CSIRO), will be published in FRDC Project No.
97/205 “Dynamics of large sessile seabed fauna important for structural fisheries habitat and biodiversity of marine ecosystems, and use of these habitats by key finfish species”.
2. The above figure is a draft and has not yet been reviewed. FRDC is awaiting the results of an external review process prior to approving this research for publication.
Given the large number of bycatch species taken in the ECTF, it is not feasible to conduct a quantitative risk assessment on each of them. A more practical approach would be to select indicator species within a risk spectrum (ranging from the most vulnerable species to the most resilient species) and assess these against the known risks (based on their distribution and abundance with respect to trawl effort). Such an approach will provide boundaries within which the ECTF should operate in order to manage the bycatch in an ecologically sustainable manner.
Recommendation 28
¾ With a view to developing a risk-assessment for ECTF bycatch species, that there be an expertise-based review of those species and that based on currently available information: (a) the species (or species groupings) taken in the ECTF be identified; and (b) their vulnerability to trawling be assessed, taking into account their known distribution and the amount of fishing effort applied.