Marine Protected Areas (MPAs) are being proclaimed around the world with the stated primary purposes of enhancing fisheries stocks and/or conserving marine biodiversity. In Australia, in response to a joint State/Commonwealth agreement to establish a National Representative System of MPAs (NRSMPA) to protect marine biodiversity, the focus is on their conservation role. However, fisheries enhancement is often suggested as an additional benefit of protection, potentially offsetting the cost of area closure in some cases.
This study aimed to contribute to the debate on the positive and negative effects of the establishment of MPAs, documenting changes that have occurred in reserves following establishment, and particularly, attempting to understand more about their role as a fisheries management tool. It builds on a program initiated following the establishment of Tasmania's first 'no-take' MPAs a decade ago.
Changes within the MPAs over the period indicated that fishing has had a substantial influence on the demographic structure of many species, particularly those targeted by fishers. The magnitude of change detected appeared to be dependant on the susceptibility of species to capture, the remoteness of protected locations and to the MPA configuration itself. Changes within the more remote Maria Island reserve (the largest area studied), relative to fished reference sites, included increases in the abundance of lobsters and certain fish species and increases in the mean size of rock lobsters (responses typical of protected areas studied elsewhere in the world), as well as a decrease in the abundance of prey species such as urchins and abalone.
Not all species increased in size and/or abundance, and for several fish species there was no significant change. At Maria Island there was also a 30% decline in the abundance of common urchins within the reserve, which may be the first Tasmanian evidence of cascading ecosystem effects related to protection from fishing. Abalone numbers were also observed to decline sharply over the period sampled. This change was interesting in that one possible explanation was an inverse relationship between predators (lobsters) and prey (abalone). If shown to be correct this finding is likely to have significant consequences for integrated, ecosystem based management of these two species.
Clearly the survey showed that MPAs, even of a relatively small size (Maria Island covers 7km of coastline), could effectively achieve conservation objectives, especially for exploited species that were resident or sedentary in nature.
A study of small-scale movement patterns of fishes showed that with few exceptions fish species showed high fidelity to site. Animals were generally resighted <100 m from initial tagging site and with individuals remaining near the tagging site throughout the 1-year duration of study. Influences on distance moved attributable to the variables body length, sex, water temperature and time since tagging were insignificant compared to variation between individuals. Patterns of movement were also generally consistent at all three study sites. Home ranges of some species were found to be affected by the presence of macroalgae, with animals emigrating from artificially cleared patches. The sedentary nature of these small- to medium-sized reef fish species indicated that relatively small marine protected areas (1 km diameter) could provide adequate protection to these fishes but suggest limited 'spillover' benefits to fisheries in the form of emigrants to surrounding areas.
The population structure of lobsters (J. edwardsii) within the Maria Island reserve after a decade of protection was substantially changed from levels prior to protection. Relative to adjacent fished areas, the abundance of females was 2.4 times greater and the abundance of legal sized females 16.8 times greater. For males these values were 4.1 and 18.6 greater respectively. The recovery of the lobster population in the Maria Island provided a reference against which the effects of fishing on a range of population biological characteristics could be examined. These included movement, growth and maturity.
Modelling the effects of MPAs was focussed on rock lobsters and abalone due to the importance of both fisheries, extensive historical datasets and differences in certain biological characteristics such as larval dispersal. Some aspects of the biology of lobsters required additional research prior to this modelling, such as the effect of increased density inside MPAs on growth, reproduction and movement. This research revealed the following:
- There was no evidence of large-scale, unidirectional migration. Tag-recaptures indicated high site fidelity with the majority of animals moving no detectable distance after periods of one to two years between capture events. This low level of movement suggested limited potential for spillover of biomass from MPAs.
- Growth in the reserve appeared slower than it was adjacent to the reserve and when closely examined it was apparent that a small proportion of the females outside the reserve were able to moult more than once in a year. It suggested that stock rebuilding inside MPAs might be slower than predicted by extrapolation of growth data collected from fished areas.
- There was a distinct spatial cline of carapace length at 50% maturity with the largest sizes being found at northwestern sites (110 mm carapace length) and the smallest sizes at southwestern sites (59 mm carapace length). This cline in size at maturity was the reverse of that described for the same species at similar latitudes in New Zealand and suggests that maturity in J. edwardsii is not primarily regulated by temperature as suggested previously.
To model the effects and implications of MPAs on fisheries, a strategy was adopted of first exploring the properties of a generally applicable simple model followed by a length-based model that was specific to the rock lobster fishery and which incorporated catch and catch rate history.
The simple model predicted population increases in both biomass and size-structure for the reserve and could be used to support the claim that, under certain constraints, a fishery managed solely through the agency of MPAs could provide a similar yield to one managed through more traditional means. However, for many species with limited larval dispersal rates the use of MPAs alone would lead to areas of relatively high quality marine environment literally surrounded by a sea of overfished and depleted areas. The more complex model highlighted a major concern when displaced effort was focused on a few of the more productive blocks. This led to these areas becoming depleted, serial displacement of catch and ultimately rapid fishery decline. It was concluded that closures displacing large amounts of abalone catch were thus a very risky management option because of a pre-disposition to serial depletion in this species.
Because abalone larval dispersal was limited, further more complex modelling was confined to rock lobster where several general conclusions became apparent. Firstly, because of the dynamics of growth and recruitment, there was a time lag before any positive effects of an MPA became apparent. The effects of large MPAs (affecting > 5% catch) tended to only become apparent after several years and the effects of small MPAs (affecting < 0.5% catch) would be hard to detect. Secondly, in an exploited population, introducing an MPA was equivalent to increasing the Total Allowable Catch or the effort outside the reserve. Introducing an MPA without reducing catch was likely to have negative effects upon most fisheries where adult movement was limited in extent, leading to reductions in total stock size and egg production. The effects would be least in lightly depleted stocks where total biomass was high relative to an unfished state. Thirdly, the impact of introducing an MPA would depend on the biology of the species concerned and the state of depletion of the stock. If the stock was already in a highly depleted state, an MPA could hasten fishery collapse. On the other hand, if a stock had already collapsed then a reserve could provide some benefit in terms of protecting mature biomass and egg production. Finally, given the assumptions of the generalized model, it appeared that it would be better to improve current management controls, in particular the match between size limits and the growth characteristics, rather than introduce large MPAs to improve the fishery.
In the Tasmanian lobster and abalone fisheries where catch and effort are effectively limited, it was concluded that the introduction of MPAs as a fisheries management tool would be inferior to present management options. Indeed, if introduced without reducing catch or effort by amounts equivalent to that in the prospective closed area, closed areas were a risky strategy that could lead to a degradation of the fishery (this appears to be a general conclusion for species with low movement rates).
Furthermore, if a fishery is being managed in accordance with ESD principles, which by definition means that the ecosystem in which it operates is not threatened by the fishery or fishing practices, then fishing should not be a key threatening process. It follows from this argument that true ESD fisheries management offers a potentially better outcome than no-take MPAs for biodiversity conservation.
This is not to suggest that MPAs do not have a place in marine and coastal management. Spatial management of fisheries has a long tradition (eg spawning grounds) and there are a number of fisheries that benefit from spatial closures. In addition they are useful where other forms of fisheries management are unavailable or poorly applied. Importantly, this study clearly demonstrates the value of MPAs as reference areas for research on the biology of exploited species and in understanding the ecosystem effects of fishing. Both add to the fisheries management toolbox and lead to a greater fisheries resource security.
Buxton CD, Barrett NS, Haddon M, Gardner C and Edgar GJ
Report to FRDC, TAFI, University of Tasmania