replenish stocks on each fishing ground. The current management strategy of fishing scallops to a catch rate threshold to ensure carryover of stock is therefore appropriate, however, the implementation of spatial closures may still be a reasonable strategy to protect spawning stock and newly settled scallops due to the lack of connectivity between fishing grounds (Kangas et al. 2012).
included species that were shown to have high, moderate and low catchability (‘vulnerability’; Kangas & Morrison 2013).
Figure 9.6. Location of sample sites in Shark Bay and two sites for depletion experiments conducted in Denham Sound. (Source: Kangas & Morrison 2013)
In total, 241 fish and 360 invertebrate species were recorded in Shark Bay during this study (see species list in Kangas et al. 2007). The 20 most abundant fish species contributed to 80 % of the total number of fish caught overall, with these species occurring at 73 – 100 % of the site sampled (Table 9.1). The 20 most abundant invertebrate species contributed up to 88 % of the total numbers of invertebrates caught and occurred at 62 – 100 % of sites sampled, except for the ascidian Herdmania pallida, which was only found at one site (Table 9.2). Seventy two species of fish (30 %) and 173 species (48 %) of invertebrates were uncommon and were only caught in one or two sites during the study (Kangas & Morrison 2013).
Table 9.1. Twenty most abundant (number per nm trawled) fish species in Shark Bay in 2002 and 2003 and proportion of sites in which they were caught (Source: Kangas &
Morrison 2013)
Table 9.2. Twenty most abundant (number per nm trawled) invertebrate species in Shark Bay in 2002 and 2003 and proportion of sites in which they were caught (Source:
Kangas & Morrison 2013)
When individual sites were examined for environmental variables, salinity attributed to the highest variation observedfor fish species, with no significant contribution of whether the site was trawled or untrawled for any of the diversity indices examined. For invertebrate species, depth was the variable providing the highest per cent variation for any of the indices, but there was some evidence that the Margalef’s richness index had a significant variation associated with whether the site was trawled or untrawled (Kangas & Morrison 2013).
Many of the individual species examined were absent or had very low abundance in the northernmost sites of Shark Bay, which had typically oceanic conditions and greater depths than the rest of the bay. These were shown to have a much higher abundance in the southern parts of Shark Bay, however, their distributions were often similar in both trawled and untrawled sites in these regions (Kangas & Morrison 2013).
Pooled data for 2002 and 2003 indicated three main groups of sites for fish and invertebrate assemblages (Figure 9.7), with each grouping containing both trawled and untrawled sites.
species much more abundant compared with other sites. This was the also only site to have extensive dense meadows of wireweed Amphibolis antarctica (Kangas & Morrison 2013).
Figure 9.7. MDS of (A) fish and (B) invertebrate species assemblages in Shark Bay for the 26 sites sampled during 2002 and 2003 (Source: Kangas & Morrison 2013)
There were significant differences in the species evenness, the Shannon’s diversity index and Simpson’s diversity index for the trawled and untrawled sites in Group 1 fish assemblages, with higher values in the untrawled sites. Significant differences were also seen in the species richness and Shannon’s diversity index for the trawled and untrawled sites within Group 3 fish assemblages, but the values were higher in the trawled sites (Figure 9.8). There were no significant differences in the diversity measures for the trawled and untrawled sites within Groups 1 and 2 for any of the invertebrate assemblages (Figure 9.9); however, there were significant differences in the species richness, the Shannon’s diversity index and Simpson’s diversity index for the trawled and untrawled sites within Group 3 invertebrate assemblages, with trawled sites having higher indices (Kangas & Morrison 2013).
The results of this study substantially expand the understanding of the impacts of trawling on the Shark Bay ecosystem and in particular, provide a detailed spatial and seasonal assessment of benthic communities, including areas where no trawling occurs. The latitudinal effects appeared to exert a stronger influence on community structure than the effects of trawling, although for fish it was shown that the fishing impacts were detectable with moderate to high trawl intensities and that low trawl effort sites had the highest abundance (Kangas &
Morrison 2013).
Figure 9.8. Least-squares mean of indices with 95 % confidence interval calculated for trawled and untrawled sits within groups identified from MDS for fish abundance in Shark bay. * indicates significant differences between trawled and untrawled sites within the grouping (Source: Kangas & Morrison 2013)
Figure 9.9. Least-squares mean of indices with 95 % confidence interval calculated for trawled and untrawled sits within groups identified from MDS for invertebrate abundance in Shark bay. * indicates significant differences between trawled and untrawled
Two depletion experiments were also undertaken in Denham Sound (sites 18 and 19) to assess bycatch species susceptibility in order to enable a comprehensive assessment for trawl impacts on bycatch species biodiversity (Kangas & Morrison 2013). At each site, the area delineated was covered by trawling north, then south with the gear overlapping the centre of the previous track so that the entire area was covered. This was repeated until all 16 sweeps were completed. Each sweep was 10 minutes in duration, and all the fish (except venomous fish in families Scorpaenidae, Plotosidae and Siganidae) and invertebrates were sorted and numbers counted after every second sweep (i.e. two sweeps sorted at a time) to even out any effects of trawling in different directions (Kangas & Morrison 2013).
For all fish species combined, the catchability was 0.23 (SE ± 0.074) per night in February and 0.06 (SE ± 0.022) in June (Figure 9.10). The low catchability overall was attributed to an increase in abundance over successive nights of the hair-finned leatherjacket (P.
choirocephalus). When this species was removed from analysis, the catchability overall for all other 46 fish species was 0.40 (SE ± 0.058). For all invertebrates catchability was 0.11 (SE ± 0.024) per night in February, with almost no decline observed in June. The low catchability in June was attributed to an increase in abundance over successive nights of a small scallop Annachlamys flabellata (Lamarck, 1819), and if this was removed from the dataset, the catchability for all the other 76 invertebrate species recorded was 0.20 (SE
± 0.037; Figure 9.10).
Figure 9.10. Depletion estimates of catchability of total fish and invertebrate species over four nights of trawling in Denham Sound (a) fish in February, (b) fish in June, (c) invertebrates in February and (d) all invertebrates, other than Annachlamys flabellata in June (Source: Kangas & Morrison 2013)
There was a significant seasonal decline in fish abundance from the period before the start of the season in February to the end of the season in November. This decline was primarily attributed to five species: L. genivittatus, P. choirocephalus, P. quadrilineatus,
sites and possibly reflects a seasonal variation in abundance and / or movement between trawled and untrawled areas. Invertebrate species abundance was only observed to decline significantly between February and June in both trawled and untrawled sites. This was attributed to the very abundant species: A. balloti, P. latisulcatus, P. rubromarginatus and H. pallida, with the first two being commercially-harvested species. As with the fish species, these four dominant invertebrate species drove the abundance patterns (Kangas & Morrison 2013).
The depletion experiment clearly indicated that some species were much more vulnerable to trawling, with catchability depending on their mobility, pattern of movement, size, physical form, burying ability, feeding mode and whether they are attracted to disturbed substrates.
Susceptible species were considered to have catchability coefficients greater than 0.6 and included the invertebrates L. maculata (sea star) and the phylum Porifera (sponges) and the fish P. octolineatus, Parupeneus chrysopleuron, L. genivittatus, S. sageneus, P. vitta, C. cephalotes and S. robusta (Kangas & Morrison 2013).
Despite a clear abundance decline in some species, no obvious impact from trawling could be discerned from abundance and distribution patterns of these species, even those with high catchability coefficients. All species recorded on trawl grounds were also present in untrawled areas. Many of the moderate- to high-catchability species that are also widespread are good candidate indicator species for trend analysis (Kangas & Morrison 2013).