Abundance

AWAITING REVIEW

Indicator summary

Summary of indicator structure and function

Definition and/or background

The following is from Fulton et al 2004a -

Changes to habitat can be reflected by changes to the communities, which inhabit them. For example, changes to species composition have been demonstrated to be a useful measure of fishing impact on habitats (Sainsbury 1987, Bianchi et al. 2000, Jennings and Kaiser 1998, Jennings et al. 2001). For example, studies in the southern North Sea revealed that increased abundance of small invertebrates indicated re-suspension of infaunal benthic organisms due to pressure waves from beam trawling (Bergman and Hup 1992, Gilkinson et al. 1998).

Attribute

Community structure, trophic structure, habitat structure and condition

Purpose

Taxa

All taxa

Data required

The following is from Fulton et al 2004a -

Long time-series of (or data from fished and unfished areas on):

• Abundance
• Biomass
• Length-frequency data
• Habitat characteristics (optional)
• Diet composition (optional)

Ecosystem applicability

The following is from Fulton et al 2004a -

Should be applicable to all ecosystems.

Robustness

The following is from Fulton et al 2004a -

Medium to high: Utility and robustness depends on the quality of the data available and the methodology used. There are some limitations in that the relative abundance (e.g. taxa in fished and unfished sites) may not reveal consistent directional changes due to disturbance. To overcome this some authors have applied Wilcoxon’s paired sign test for clarification (Kaiser and Spencer 1996). If using long-term time-series the degree of change may be underestimated if the time-series does not begin until there have already been substantial changes in the area being monitored. In addition, other environmental or anthropogenic pressures on a system (e.g. ENSO or eutrophication) could also affect abundance.

Current status and trends

The following is from Fulton et al 2004a -

Decreased abundance

In experimental studies of trawling on stable sediments at depths of between 27- 40m in the Irish Sea, Kaiser and Spencer (1996) recorded that the number of species and individuals was reduced by two and threefold respectively (Jennings and Kaiser 1998). Elsewhere, scallop dredging in the Clyde Sea area (Scotland) led to a 70% reduction in abundance (by number) of counts of live maerl (coralline algae) thalli with no sign of recovery after 4 years (Hall-Spencer and Moore 2000). Maerl beds form extremely long-lived complex sediments and create areas of high biodiversity, so their loss can have significant community-level impacts. In Australia it was found that the abundance of fish and small invertebrates increased in closed areas and decreased in fished areas of the North West shelf (Sainsbury et al. 1997). The fish community changed over an 11 yr period from larger ambush predators (Lutjanus and Lethrinus), which inhabit structurally complex habitats dominated by large epibenthic fauna such as sponges, to smaller fish typical of open habitats (Saurida and Nemipterus). A model relating the fish species abundance to habitat structure showed that this change was most likely due to destruction of large epibenthic sponges, alcyonarians and gorgonians by trawl gear that provided cover, shelter and food for the displaced species. (Sainsbury et al. 1997). Another study considering fish community was carried out on the Georges Banks, North Atlantic. It was found that the fish community structure changed from gadoids and flounders to small elasmobranchs (dogfish and skates) (Fogarty and Murawski 1998). This change was an indirect effect of reduction of the abundance of gadoids and flounders by fishing, which in turn reduced competition for the replacement species. The second order consequence may be an impact on foraging seabirds and mammals which utilised the gadoid and flounder as prey.

Use of abundance as an indicator in assemblage analysis

Abundance trends of species assemblages and /or sensitive taxa can indicate habitat disturbance or recovery. For example, fish assemblages that are indicative of particular benthic habitat types have been determined for some fisheries, including: deep water slope communities off SE Australia (Bax et al. 1999); in protected areas with some fishing in the Stellwagen Bank National Marine Sanctuary, in the continental shelf region of the Gulf of Maine, Northwest Atlantic (Auster 2001); in no-take marine reserves that are recovering from fishing in Tasmania (Edgar and Barrett 1999); and for the northern prawn fishery in tropical Australia (Stobutzki et al. 2001).  Changes in these assemblages over time may provide reference points of fishing pressure. The use of these indices has also been demonstrated with invertebrate groups (Collie et al. 2000) and elasmobranch species (Stevens et al. 2000).

Change in relative abundance from No-Take marine reserves as reference points

Data on the relative abundance gained from studies of communities in no-take reserves could provide important thresholds of recovery levels for some species and communities for future recovery of fished reefs. For example, assemblages from a recovering sub-tidal southern hemisphere temperate reef system have been determined in Tasmania’s marine reserves (Edgar and Barrett 1999) after the areas were closed to all forms of take over 5 years.

Abundance and distribution of indicator species following multivariate analysis to identify species vulnerable to fishing

Jennings and Reynolds (2000) and others (Jennings et al. 2001) have reviewed metrics such as diversity indices for determining fishing effects on communities and have concluded that identifying and measuring species that discriminate between fished and unfished communities is a reliable, simple and cost effective method of monitoring or identifying fishing impacts on ecosystems. A variety of studies in varying marine ecosystems around the world have now begun determining indicator species (e.g. Fulton and Smith in press) and once indicator species have been identified, they could be measured over time by monitoring abundance and distribution. Applicable methods include:

• % abundance changes
• Coefficient of variation of abundance
• MDS and Bray-Curtis similarity indices (these are the recommended multivariate methods for teasing out species associated with particular habitats)

Metrics applied to measure community data must be powerful enough to detect subtle changes and be sensitive to changes in rare or vulnerable taxa. In investigating impacts of fishing on benthic communities, many authors have used combinations of univariate and multivariate analysis and measures (Collie et al 1997, Tuck et al 1998Collie et al 2000, Bianchi et al 2000). These commonly include abundance, density, biomass, length (fish), diversity, cluster analysis, similarity indices (e.g. Bray-Curtis similarity index), Multidimensional scaling (MDS) (for identifying shifts in community composition) and a variety of tests for statistical significance (e.g. ANOVA’s, pair-wise t-tests, and Wilcoxon’s paired sign test).

If time-series are used to detect vulnerable species, rather than comparisons with unfished areas, then long time-series data are required. An example of this approach is from the North Sea. In an extended study on a data series to describe diversity changes between 1925-1996 in Scottish North Sea using Hill’s indices and k-dominance curves (Greenstreet et al., 1999), it was found that species diversity declined in areas of high fishing intensity. It was also found that fishing effects caused the largest decrease in abundance in skates and rays, which are K-selected species that are highly vulnerable to exploitation (Greenstreet et al., 1999). Studies of trawling impacts in the Dutch sector of the North Sea showed that overall, trawling resulted in lower densities of vulnerable species and a greater abundance in opportunistic species (Eleftheriou 2000). For fish, a study aimed at ranking 411 tropical bycatch species of 99 families in the Australian Northern Prawn Fishery with respect to their vulnerability to trawling (Stobutzki et al. 2001) found that the most vulnerable species (high susceptibility to capture and mortality) were mainly benthic or demersal, closely associated with the sea floor, from primarily soft or muddy sediments, and with a diet that includes prawns. Mortality or injury rates of vulnerable fish species may be potential measures of trawl impacts on these species, but it needs further testing. Vulnerability rankings have been determined for bycatch fish and catch thresholds for the vulnerable fish could be determined using the q jeopardy model (Pope et al), which could provide an index of catch limits for the species or aggregate of species.

References

Fulton, E.A., Smith, A.D.M., Webb, H., and Slater, J. (2004a) Ecological indicators for the impacts of fishing on non-target species, communities and ecosystems: Review of potential indicators. AFMA Final Research Report, report Number R99/1546.

References that Fulton et al uses for this indicator:

Auster, P. J., and R. W. Langton. 1999. The effects of fishing on fish habitat. American Fisheries Society Symposium  22: pp 150-187.

Bax, N., A. Williams, S. Davenport, and C. Bulman. 1999. Managing the ecosystem by leverage points: a model for a multispecies fishery. In: Ecosystem approaches for fisheries management: proceedings of the Symposium on Ecosystem Considerations in Fisheries Management, pp 283-303. University of Alaska Sea Grant College Program, no. AK-SG-99-01. Fairbanks, Alaska: University of Alaska Sea Grant Program.

Bianchi, G., H. Gislason, K. Graham, L. Hill, X. Jin, K. Koranteng, S. Manickchand-Heileman, I. Paya, K. J. Sainsbury, F. Sanchez, and K. Zwanenburg. 2000. Impact of fishing on size composition and diversity of demersal fish communities. ICES Journal of Marine Science 57: pp 558-71.

Bergman, M. J. N., and M. Hup. 1992. Direct effects of beamtrawling on macrofauna in a sandy sediment in the southern North Sea. ICES Journal of Marine Science 49: pp 5-11.

Collie, J. S., S. J. Hall, M. J. Kaiser, and I. R. Poiner. 2000. A quantitative analysis of fishing impacts on shelf-sea benthos. Journal of Animal Ecology 69: pp 785-98.

Edgar, G. J., and N. S. Barrett. 1999. Effects of the declaration of marine reserves on Tasmanian reef fishes, invertebrates and plants. Journal of Experimental Marine Biology and Ecology 242: pp 107-44.

Eleftheriou, A. 2000. Marine benthos dynamics: environmental and fisheries impacts: introduction and overview. ICES Journal of Marine Science 57: pp 1299-302.

Fogarty, M. J., and S. A. Murawski. 1998. Large-scale disturbance and the structure of marine systems: fishery impacts on Georges Bank. Ecological Applications 8, no. 1, Supplement: pp S6-S22.

Fulton, E.A., and A.D.M. Smith. 2004.  Lessons learnt from the comparison of three ecosystem models for Port Phillip Bay, Australia. South African Journal of Marine Science

Gilkinson, K. D., M. Paulin, S. Hurley, and P. Schwinghamer. 1998. Impacts of trawl door scouring on infaunal bivalves: results of a physical trawl door model/dense sand interaction. Journal of Experimental Marine Biology and Ecology 224: pp 291-312.

Greenstreet, S. P. R., F. E. Spence, and J. A. McMillan. 1999. Fishing effects in northeast Atlantic shelf seas: patterns in fishing effort, diversity and community structure. V. Changes in structure of the North Sea groundfish species assemblage between 1925 and 1996. Fisheries Research 40: pp 153-83.

Hall-Spencer, J. M., and P. G. Moore. 2000. Scallop dredging has profound, long-term impacts on maerl habitats. ICES Journal of Marine Science 57: pp 1407-15.

Kaiser, M. J., and B. E. Spencer. 1996. The effects of beam-trawl disturbance on infaunal communities in different habitats. Journal of Animal Ecology 65: pp 348-58.

Jennings, S., and M.J. Kaiser. 1998. The effects of fishing on marine ecosystems. Advances in Marine Biology 34: pp 201-351.

Jennings, S., and J. D. Reynolds. 2000. Impacts of fishing on diversity: from pattern to process. In: The effects of fishing on non-target species and habitats. Editors M. J. Kaiser, and S. J. de Groot, pp 235-50. 399 p . Oxford: Blackwell Science.

Jennings, S., M.J. Kaiser, and J.D. Reynolds. 2001.  Marine fisheries ecology.,. 417 p . London: Blackwell Science.

Pope, J.G., D. S. MacDonald, N. Daan, J. D. Reynolds, and S. Jennings. 2000. Gauging the impact of fishing mortality on non-target species. ICES Journal of Marine Science 57: pp 689-96.

Sainsbury, K. J. 1987. Assessment and management of the demersal fishery on the continental shelf of northwestern Australia. In: Tropical snappers and groupers: biology and fisheries management. Editors J. J. Polovina, and S. Ralston, Chapter 10-465-502. Ocean Resources and Marine Policy Series. Boulder, Colorado: Westview Press.

Sainsbury, K.J., R. A.Campbell , R. Lindholm and A.W. Whitelaw 1997. Experimental management of an Australian multispecies fishery: examining the possibility of trawl-induced habitat modification. In: Global Trends: Fisheries Management, Editor(s) E.K. Pikitch, D.D. Huppert, and M.P. Sissenwine, pp 107-112. American Fisheries Society: Bethesda, Maryland.

Stevens, J. D., R. Bonfil, N. K. Dulvy, and P. A. Walker. 2000. The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems . ICES Journal of Marine Science 57: pp 476-94.

Stobutzki, I., M. Miller, and D. Brewer. 2001. Sustainability of fishery bycatch: a process for assessing highly diverse and numerous bycatch. Environmental Conservation 28, no. 2: pp 1-15.

Tuck, I. D., S. J. Hall, M. R. Roberston, E. Armstrong, and D. J. Basford. 1998. Effects of physical trawling disturbance in a previously unfished sheltered Scottish sea loch. Marine Ecology Progress Series 162: pp 227-42.

Background reading

Fulton, E.A., Fuller,M., Smith, A.D.M., and Punt, A. (2004) Ecological indicators of the ecosystem effects of fishing: Final report. AFMA Final Research Report, report Number R99/1546.

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