Principal Investigator: Andrew Tobin
Keywords: Spanish mackerel, spawning aggregation, marine protected area, Great Barrier Reef, acoustic monitoring, oral histories
In response to increasing concerns from long-term fishers, and fisheries (QDAFF) and conservation (GBRMPA) managers of the Queensland east coast Spanish mackerel fishery (ECSMF), this project explored and defined historic patterns of exploitation, contemporary patterns of vulnerability and methods for mitigating risk. Project staff from the Centre for Sustainable Tropical Fisheries and Aquaculture (CSTFA) and University of Queensland (UQ) reconstructed a 63 year timeline (1949-2012) of trends in the fishery that allowed contemporary productivity to be compared to historical productivity. During 2010 and 2011 innovative acoustic monitoring techniques were used to identify the current day characteristics of Spanish mackerel aggregating to spawn around a discrete group of well-known spawning reefs off the Townsville coast. These methods were used to identify if the current marine park zoning plan, that includes the protection of some historically important Spanish mackerel spawning reefs, offers any protection to mackerel from fishing. Finally, a management strategy evaluation modelling exercise explored the applicability of spatial and temporal closures as a management tool to reduce catch and/or increase protection of Spanish mackerel aggregating for the purposes of spawning.
Spanish mackerel support important fisheries within Queensland, with a commercial fishery that has operated for in excess of 100 years and an ever important and ever-growing recreational sector. The latest stock assessment of the Queensland east coast Spanish mackerel fishery indicates a fully fished and possibly over-fished status (Campbell et al 2012). This is concerning given only about half of the commercial TAC has been caught in recent years, and contemporary recreational fishery effort and catch is ever increasing. In adding to this stock status concern, anecdote from long-term fishers suggests that spawning aggregations of Spanish mackerel have notably declined in both space and time over a long period of time suggesting the reproductive capacity of the Queensland stock is and has been diminishing over a considerable period of time.
To better understand the vulnerability of Spanish mackerel to fishing, a recent quantitative assessment of commercial logbook data was completed (Tobin et al 2013). Complementary data analyses explored data trends and identified that each year the Queensland east coast Spanish mackerel stock that forms large and predictable spawning aggregations for a very short and defined period (October and November) in a very small well known area of the central Great Barrier Reef (GBR)(Tobin et al 2013) and these aggregations support a disproportional amount of fishing effort and catch. This type of spawning aggregation behaviour exhibited by Spanish mackerel is termed transient aggregation and history demonstrates that is characteristics means species are extremely vulnerable to overexploitation, rapid stock depletion and in severe cases extirpation of local stocks.
The aims of this project are therefore threefold – 1. to quantify the historical changes that have occurred in the spawning aggregation fishery; 2. use acoustic monitoring techniques to understand the aggregating behaviours of spawning Spanish mackerel and whether or not the current Marine Protected Area network of the GBR offers some protection from fishing; and 3. complete an oral history and historical archives search to recreate the performance trends of the ECSMF in order to better understand the state of the contemporary fishery.
The project employed three methods to address the three complementary objectives. To map the historical trends in fishery catch both through space and time, we explored both contemporary logbook data (1988-2012) as well as historic records (1949-2012) sourced through newspaper archives, as well as personal logbooks and oral histories from current and retired fishers. These methods allowed an understanding of trends that have occurred within the spawning aggregation fishery over the extensive history of the fishery. To better understand the aggregating behaviours of Spanish mackerel during their spawning season we used innovative acoustic monitoring technology. Up to thirty-eight acoustic receivers were deployed among 13 reefs where spawning aggregations and fishing have long occurred northeast of Townsville. Across two spawning seasons (2010, 2011) 105 individual Spanish mackerel were tagged with uniquely coded acoustic transmitters that allow individual as well as aggregating behaviours to be detected and described. Finally, a management strategy evaluation was conducted by imposing theoretical closures on contemporary catch data trends to model the impacts of various forms of temporal closure.
The reconstruction of the historical Spanish mackerel fishery clearly identified the Queensland east coast Spanish mackerel fishery has declined through time. The description of extirpation of historically important spawning aggregations from reefs east of Cairns as well as a reduction in size and frequency of Townsville reef aggregations is particularly worrying. In combination these factors suggest the overall size and thus reproductive potential of the east coast stock has been significantly reduced. The acoustic monitoring component of the project identified a remarkable aggregating behaviour of Spanish mackerel during their annual spawning season. This behaviour included very strong reef and aggregation fidelity for a period of a single lunar month, after which mackerel appeared to disperse from the spawning reefs. Very few between reef movements were detected suggesting a possible strong homing reef ability and behaviour. Such a defined aggregating behaviour suggests that spatial closures are likely to be effective at protecting some Spanish mackerel during spawning. The acoustic monitoring was not able to identify which reef or reefs out of a complex of 13 reefs recognised as important for spawning supported the largest and strongest aggregations. While contemporary fisherman’s local ecological knowledge as well as oral history records suggests Rib Reef supports the largest and longest-lasting aggregation of each season, neither the acoustic the tagging nor the contemporary logbook data where able to confirm this. This type of reef importance definition could have provided managers with specific recommendations of spatial closures should that type of management become necessary.
The implications for the commercial fishing industry are significant. The key findings highlight a significant contraction of a fishery resource that has historically supported a considerable level of commercial effort and harvest. Under current stock conditions, management policies and increasing recreational participation, there are no positives regarding the historically important spawning aggregation fishery. Future investment in the commercial sector should be made with upmost caution. The recreational fishery should take particular note of the stock status, historical trends and increasing participation within this fishery and consider the near, medium and long-term consequences. Similarly managers should treat with considerable caution any increases in effort and catch in the Queensland east coast Spanish mackerel fishery while the stock status remains critical. The most difficult question for all stakeholders in how should this finite resource be shared in future years while current trends suggest an increasing desire to access the ECSMF by recreational fishers while traditional commercial fishing seasons and grounds support fewer fishers each year.
In addition to the clear outcomes of this research, we recommend a number of areas for future research, monitoring and management effort. Firstly, a limitation of this research was the inability to define the relative importance of each reef within the spawning complex to Spanish mackerel aggregating to spawn. Research sampling as well as commercial and recreational fishing effort clearly shows that each reef supports an aggregation of mackerel. It may be possible the reef aggregations vary markedly in size and thus spawning potential. Identifying those reefs that are disproportionately important to the aggregating mackerel is paramount should further spatial protection of spawning aggregations of Spanish mackerel be considered necessary. Secondly, the analysis of commercial fishery data through the extensive history of the Spanish mackerel fishery clearly identifies a fishery in continual decline in catch, effort and fisher participation. In contrast, participation in the recreational fishery continues to increase. Any future research, monitoring and management needs to explicitly consider the characteristics of the recreational fishery which in the current day is likely to exceed commercial effort and catch. Contemporary and robust estimates of recreational catch are urgently needed. Further monitoring efforts should also include the identification of fishing effort and catch “hotspots” of the recreational fishery. As has been demonstrated by Tobin et al (2013) and the outputs from this project, Spanish mackerel are an obligate transient aggregator and this behaviour suggests temporal and/or spatial closures could be an effective effort and catch constraining management tool should that type of management intervention be required. When and where effort and catch peaks for Spanish mackerel is likely to be indicative of aggregating behaviours that render Spanish mackerel highly vulnerable to fishing. This knowledge could however also assist managing future catch and effort through spatial and/or temporal closures. Finally, as the participation and catch trends within the commercial and recreational fishing sectors appear negatively correlated since the mid-2000s, methods for managing this changing allocation of the resource need to be considered.