Final Report - 2013-028-DLD - Review and update harvest strategy settings for the Commonwealth small pelagic fishery
: Anthony D.M. Smith, T.M. Ward, F. Hurtado, N. Klaer, E. Fulton, A.E. Punt
This study undertook ecosystem and population modelling to evaluate and provide advice on the reference points (e.g. biomass depletion levels) and settings (e.g. exploitation rates) for the four main target species in the harvest strategy of the Commonwealth Small Pelagic Fishery (SPF) – Jack Mackerel, Trachurus declivis, Redbait Emmelichthys nitidus, Blue Mackerel Scomber australasicus and Australian Sardine Sardinops sagax.
The study used a new variant of the Atlantis ecosystem model (Atlantis-SPF). The model found that both singly and in combination, depleting these target species has only minor impacts on other parts of the ecosystem. Unlike some other regions which show higher levels of dependence on similar species, such as in Peru and the Benguela systems (Smith et al., 2011), the food web in southern and eastern Australia does not appear to be highly dependent on SPF target species. None of the key higher trophic level predators in SE Australia, such as seals, penguins and tunas, have a high dietary dependence on these species. Studies using other ecosystem models such as Ecosim in the same region have reached similar conclusions (Goldsworthy et al., 2013; Bulman et al., 2011).
The findings have implications for the target and limit reference points that should be selected for the main commercial species in the SPF. Equilibrium BMSY for these species ranged from about 30 to 35% of unfished levels. However, these levels are uncertain and it may be more appropriate to use the default values from the HSP with BMSY set at B40 (40% of unfished levels) and the default BMEY set at 1.2 times this level, close to B50. This study suggests that the target reference point for these SPF target species should be set at B50 and the limit reference point at B20, in line with the HSP default settings. The results presented in this report, combined with evidence from other studies, suggest that these levels are safe from an ecosystem perspective and provide reasonable levels of yield relative to MSY.
Population modelling suggests that target exploitation rates (ERs) for the SPF should be species-specific and possibly even stock-specific. The current average Tier 1 harvest rate of 15% appears to be too high for eastern Redbait. Taking account of some of the sensitivity scenarios, the report states that it may also be too high for western Redbait and Jack Mackerel.
The results help inform the choice of suitable ERs for each of the species and stocks. For Tier 1, the analyses focus on achieving the reference points recommended by the ecosystem modelling, that it is to achieve a median depletion of 0.5 or B50, while maintaining less than a 10% chance of falling below the suggested limit reference point of B20. The base case exploitation rates that achieve this target, assuming surveys every five years, are as follows:
- Eastern Redbait 9%
- Western Redbait 10%
- Jack Mackerel 12%
- Eastern Blue Mackerel 23%
- Western Blue Mackerel 23%
- Eastern Sardine 33%
- Western Sardine 33%
In the current harvest strategy Tier 2 rates are set at half the Tier 1 rate. It was assumed that the Tier 2 rate would only be applied after 5 years of exploitation at Tier 1, and that no further surveys would take place. The report summises that it is generally not safe to apply Tier 2 for long periods of time unchecked. Particularly for the shorter lived species (Blue Mackerel and Sardine), this can result in unacceptable probabilities of depletion in quite short periods of time (5 or 6 years), while the period is on the order of 20 years for the other two species. The study found that an alternative approach would be to make the Tier 2 rate more precautionary (i.e. less than half the Tier 1 rate) and/or reduce the period over which it is applied (e.g. not more than 5 years).