McGrath, SP 2012, 'Physical impacts of recreational terminal fishing rigs in NSW, Australia', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright SP McGrath 2012
Globally, recreational fishing is one of the most popular leisure activities with participation rates estimated at > 40% in some developed countries. Such high participation put more strain on an already depleted fisheries resource; however, while fishing pressure has increased so has the number of released individuals. This is due to anglers releasing at least part of their catch, either voluntarily or in response to regulations that include legal sizes and personal quotas. Australia is no different with a combined release rates of > 30% for all species. More specifically, three of the most important coastal species targeted by recreational anglers are mulloway (Argyrosomus japonicas), yellowfin bream (Acanthopagrus australis) and snapper (Pagrus auratus), with > 11 million individual fish released annually. However, there are many concerns regarding their post release fate and the consequences of catch-and-release angling. Given this, two field and five aquaria experiments were completed to quantify impacts associated with current catch-and-release methods.
The two field experiments were completed to identify and assess the occurrence of factors associated with the post-release mortality of angled P. auratus. The results from these experiments support the general consensus from previous work that hook ingestion is the primary factor influencing mortality, although the proportion of fish affected can be minimised by simply cutting the line and releasing the fish. However, the utility of this approach is based on the premise that hook-ingested individuals eventually eject their hooks, often after sufficient decay. The third experiment (initial aquaria based experiment) aimed to identify the technical parameters that contribute to increased hook decay by exposing different hook designs to seawater to investigate temporal and structural degradation. Degradation was mainly influenced by the hook material and wire diameter. The fourth experiment involved selecting three hook types (from the previous experiment) with similar absolute sizes made from different materials (stainless steel and nickel-plated and red-lacquer carbon steel) and then either modified (using small notches) or left unmodified. Individuals of the three species of fish listed above were then allowed to ingest one of the selected hook types (during angling), to determine the rate of hook ejection and their long-term fate. Hook ejection was the primary factor that influenced mortality in all three species, with only one death among those fish that ejected their hooks (133 fish in total) compared to almost half of those that retained their hooks. This research supports the use of hooks that oxidise faster as they subsequently become weaker and are ejected more readily. However, ingested nickel-plated hooks could be detrimental to fish health as they resulted in significantly more deaths among A. australis and A. japonicus when compared to the other two hook types. This was investigated further in the final experiment which quantified the absorption of metals by A. japonicus that ingested nickel-plated hooks. Nickel-plated hooks resulted in elevated concentrations of nickel in the liver and blood, which were significantly higher than wild and control individuals.
This thesis demonstrates that hook ingestion results in increased mortality, and that cutting the line on angled fish increases their chance of survival as they will eventually eject their hooks. Hook ejection also augments survival; however, this is strongly influenced by hook decay. This may prove problematic, as the increased metal concentrations may negatively impact fish health. Therefore, future research should aim to investigate and develop hook designs that degrade quickly once immersed in seawater, which would inevitably increase the rate of hook ejection and reduce fish mortalities.