Patterns In Abundance And Size Of Sharks In Northwestern Australia: Cause For Optimism
A growing human population has increased pressure on commercial fish populations to meet the growing food demand. Sharks are not escaping this high fishing pressure, and fishing of sharks has generated global concern around ongoing shark conservation efforts. Articles reporting shark population declines are receiving a lot of media attention, but what if there was some good news about shark populations?
The State of Things
The general perception is that shark populations are globally at risk and that sharks cannot survive an increase in fishing pressure. A decline in apex predators, such as sharks, can alter food webs, and therefore it is important to understand population trends of apex predators to ensure conservation and foster an understanding of ecosystem functioning. Complex models are often used to quantify the effects of fishing exploitation on apex predators, however these models can only be applied to species that have extensive data available. Sharks are mostly taken as incidental catch and are not targeted species. It is therefore not surprising that stock assessments are only available for a small fraction of shark species populations.
Sharks in Western Australia
Several shark species inhabit the waters of Western Australia (WA) and most of these species have been captured by a range of fisheries as bycatch. Commercial shark fishing started in the 1940s, and catch rates increased into the early 2000s. As a result, catch limits and area closures were introduced in the late 2000s to protect shark species. A large area of northwest WA (~ 0.8 million km2) was closed to commercial fishing of sharks and rays in 1993 and 2005. This area closed to fishing was established to protect breeding stocks of dusky sharks and sandbar sharks; species which were prone to over-exploitation.
It is crucial for conservationists and marine biologists to understand temporal and spatial patterns in shark populations, as sharks play an important role in regulating marine ecosystems. These patterns also play a role in assessing the impacts of fishing at population level, to ensure sustainable harvesting of marine resources.
Unfortunately, long-term information on abundance and average size of sharks is limited, especially from sources independent of fisheries. Most available data come from commercial fishing catch rate information, which makes the assumption that catch rates are proportional to abundance. This assumption has repeatedly been proven as being incorrect, due to changes in fishing efficiencies.
A study published in ICES Journal of Marine Science in 2019 attempted to override these assumptions by analyzing information collected from scientific surveys between 2002 and 2017 that was undertaken in an area closed to fishing in northwestern Australia, in an attempt to determine spatial and temporal patterns in standardized catch rates and average sizes for numerous shark species. This study aims to better understand the population status of these sharks, independent of catch rates in the fishing industry.
Results of the Study
The study used scientific surveys to determine and monitor patterns in catch rates and size of sharks along northwestern Australia. Sampling occurred in an area that was closed to commercial fishing in stages between 1993 and 2005. Despite widely reported declines in shark catch rates due to unmanaged fishing globally, the catch rates of northwestern Australian sharks remained stable. Sandbar sharks were the most commonly sampled species in the survey, and although there was an initial decline in population numbers between 2002 and 2008, there was an increase in this species following the 2005 closure.
Many of the sampled species occur throughout WA and have been reported as both targeted and accidental catch. In particular, sandbar and dusky sharks were two of the main commercial shark species historically fished in the waters of WA. The latest assessment of these shark species indicates that populations are recovering following commercial fishing, but it could take several decades for the populations to fully recover.
Other shark species that were analyzed showed fluctuating, but stable, catch rates. This stable pattern suggests that either no major fishing impacts on populations prior to establishments of fishing closures, or low statistical power in sampling design for determining patterns for species with low abundance in the study area. For many of the shark species present, historic catches have always been relatively low, therefore a lack of catch rate is unlikely to indicate non-recovery from a depleted population. More research is required to combine information from the study with the life history and catch of these other shark species to determine their population statuses.
For all shark species, mean size fluctuated across years, not showing any particular trend. Generally, it is expected that the mean size of individual sharks in a harvested population would decrease, as larger individuals are removed from the population. For the sharks reported in the study, mean size remained stable.
Though currently not a common trend, there are cases of positive outcomes from management intervention (such as fisheries closures) on shark populations. This indicates that using science-based management, shark stocks have the potential to support sustainable shark fisheries (both as a target species and bycatch). Considering that according to IUCN assessments one-quarter of all sharks and rays are threatened with extinction due to overfishing, the study offers some optimism for shark populations, at least in Australia.
A Brighter Future
It is unlikely that the bycatch of sharks in commercial fisheries will ever be completely prevented. Instead, well-managed sustainable fishing practices have been suggested as a viable option to ensure the survival of shark populations. Fishing closure areas may be particularly beneficial in shark management and conservation, as long as the fishing effort is reduced (not just displaced to another area).
Fishing closures cannot be used in isolation, especially when it comes to conserving and managing wide-ranging species that spend time outside of closure areas. Effective shark conservation will require a variety of management tools and enforcement by government regulations. Current measures implemented in WA, such as fishing closures, gear inspections and vessel monitoring systems, prove that science-based management can achieve a sustainable fishing industry, while allowing shark populations to recover to meet conservation objectives.