Climate Change Impact and Structural Reforms in Kiribati
KIRIBATI
Box 1. Assessing Sustainability in Fisheries Management
The concept of maximum sustainable yield (MSY) is central to fisheries management. MSY is the largest
average catch that can be taken from a stock under average environmental conditions without affecting stock
health. Catch statistics per se are not a reliable indicator of depletion or abundance. Related indices frequently
used as reference points within the WCPFC are:
⚫ F/FMSY, an indicator for the risk of becoming overfished—the level of current fishing mortality (F) compared
to the fishing mortality at maximum sustainable yield (FMSY). When F/FMSY > 1, overfishing of the stock is
occurring and the rate at which fish is caught is not sustainable in the long term.
• SB/SBF=0, an indicator for stock depletion or overfishing—the current amount of spawning biomass (or adult
tuna quantity) (SB) compared to the estimated amount of adult fish that would be present within the stock if it
had not been fished (SBF-0). A limit reference point of 20 percent SBF=0 has been adopted for key tuna stocks, as
it is considered a level below which the health of tuna stocks will be affected. For skipjack, a higher limit
reference point in the 40-60 percent range has been proposed, but this has not been adopted.
B/BMSY, a variant for stock depletion, compares a stock's abundance or biomass (B) against its level at
maximum sustainable yield (BMSY).
In addition, an environmental impact rating measuring bycatch is used to assess sustainable fishing. This
measures the amount of any species caught that is not targeted by the boat.
Alternative, broader indicators of marine ecosystem integrity and sustainability at the global and regional levels
are the Marine Trophic Index (MTI) and Fisheries in Balance index (FIB). More specifically:
Trophic level (TL) is defined as the position of an organism in the food chain and ranges from a value of 1
for primary producers to 5 for marine mammals and humans (Pavluk and bij de Vaate 2017). The trophic level of
most fishes can have any value between 2.0 and 5.0, and changes through the life history of fish, with juveniles
having lower trophic levels than adults. A falling TL may indicate an increase in fish mortality as it results in more
adult fish caught.
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The FIB index that is stable reveals whether changes in trophic levels are matched by appropriate changes in
the catch in the opposite direction. FIB increases if both catches and mean trophic level increase, for example
due to higher fish biomass or geographic expansion, suggesting that the fishery was expanding to stocks
previously not, or lightly exploited. FIB will decrease when TL shows a stepwise decline and a corresponding
increase in catches.
10.
The WCP skipjack tuna, commercially the most important tuna species, does not
appear overfished and the fishing rate is sustainable, although some studies have point to
concerns. Based on the PNA Office data, the skipjack stock depletion appears to have increased,
with SB/SBF-0 going down from nearly 90 percent in the early 1970s when tuna fisheries just started
to around 45 percent in 2019-significantly above the official 20 percent limit reference point-but
close to the lower bound of the unofficial 40-60 percent limit reference point. The Bigeye tuna stock
depletion shows a similar path over time, coming down from 95 percent in 1960 to 40 percent
in 2019. The Yellowfin and Albacore tuna stock depletion metrics appear more favorable, with
SB/SBF=0 close to 60 percent in 2019, also down from 90 percent in the 1960s. Other sources, such as
the United Nations Food and Agriculture Organization find that most tuna stocks are fully exploited,
meaning there is no room for fishery expansion, and some are already overexploited, i.e., at risk of
stock collapse. According to the International Seafood Sustainability Foundation, 65 percent of tuna
stocks are at a healthy level of abundance, but 13 percent are considered overfished. The United
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