methodology - Forest and Bird

BMEY – the recruited biomass that supports the maximum economic yield.
ESY – Ecologically sustainable yield – the yield an ecosystem can sustain without shifting to an
undesirable state.
MSY - Maximum sustainable yield, in relation to any stock, means the greatest yield that can be
achieved over time while maintaining the stock's productive capacity, having regard to the
population dynamics of the stock and any environmental factors that influence the stock:
(section 2, Fisheries Act 1996)
OSY – Optimum sustainable yield is the yield which considers factors in addition to maximum
sustainable yield, for instance, effects on other species in the ecosystem and of other human
uses of the ecosystem.
Biological reference points can be used to assess the sustainability or state of a stock, for example, the
percentage of the unfished population remaining or the portion of eggs or pups produced when compared
to the unfished population. In New Zealand quantitative stock assessments the current biomass of a stock
is usually compared to the unfished, unexploited or virgin biomass (Bo) (Cordue 2004, Ministry of Fisheries
2008). New Zealand’s harvest strategy also includes limit reference point on fishing mortality (e.g. FMSY).
Reference point catch limits (e.g. current annual yield (CAY)) rely on agreement as to what constitutes an
acceptable level of risk of stock decline below a certain size. These are usually defined in New Zealand
stock assessments which state that the biomass should not have the risk of falling below 20% of unfished or
virgin biomass (Bo) more than 10% of the time. This is known as the safety condition (Francis 1992 and
1999). As Francis (1999) noted “without this condition, BMAY and BMCY for southern blue whiting are 15% Bo
and 17% Bo respectively…with the safety condition they are 29% Bo and 49% Bo”.
The Ministry of Fisheries 2008 harvest strategy sets:
“a specified target about which a fishery or stock should fluctuate; a soft limit that triggers the
requirement for a formal, time-constrained rebuilding plan; and a hard limit below which fisheries
should be considered for closure” (Ministry of Fisheries 2008a).
However, there are elements of the strategy which may not be consistent with the legislative requirement
to keep stocks at or above the MSY level (section 13(2)(a)).
“The harvest strategy standard does not explicitly specify a %Bo target, and alternative %Bo targets
will be acceptable, provided they can be adequately justified by, for example, considerations of stock
productivity. However, it is becoming increasingly difficult to justify stock targets less than 30-40% Bo
(or, equivalently, removing more than 60-70% of the unfished biomass).” (Ministry of Fisheries, 2008).
In contrast the Australian harvest strategy (DAFF 2007) sets a proxy for BMSY at 40% Bo.
Retaining stocks closer to their unfished size has been the outcome of the focus on precautionary fisheries
management and including ecological considerations.
Ecological impacts of reducing fish stocks on prey species or predator species have been important
consideration of CCAMLR’s approach to fisheries management. In CCAMLR target levels have been set at
50%Bo for predator species (e.g. toothfish) and 75%Bo for prey species (egg krill or icefish) (Constable et
al.,., 2000).
Reducing the impact on ecosystems and seabird predators has led to further support for retaining stocks at
much higher biomass levels than Bmsy. Smith et al., 2011 noted that retaining stocks at higher levels (egg
75%Bo) for low trophic level species had lower ecological impacts. They noted that “halving exploitation
rates would result in much lower impacts on marine ecosystems while still achieving 80% of MSY”.
Maintaining larger fish stocks also ensures the retention of larger older fish. Larger older female fish
produce many more eggs that younger smaller females. The big old fat fecund female fish (BOFFF)
Forest & Bird Best Fish Guide 2012 – 2013: Criteria for Ecological Rankings 9