Shelton J. Harley

Fisheries: Decline of Pacific tuna populations exaggerated?

industrial fisheries in the Pacific Ocean and elsewhere since the 1950s. In their analysis of Japanese longline-fishery catchper-unit-effort (CPUE) data, Myers and Worm conclude that the community (species-aggregated) biomass of large pelagic fish, mainly tunas, was reduced by 80% during the first 15 years of exploitation and is now at 10% of pre-industrial levels. We show here that an assumption critical to this conclusion — namely, that Japanese longline CPUE acts as an accurate index of community biomass — is invalid. Our results indicate that biomass decline and fishing impacts are much less severe than is claimed by Myers and Worm. Interpretation of the species-aggregated CPUE as an index of community biomass rests on the assumption that catchability (a coefficient specifying the proportionality between CPUE and abundance) is constant across species and over time. The former is unrealistic because, among other things, the species have different depth distributions and hence different vulnerability to longline gear. The evolution of tuna longline fisheries in all oceans has seen changes in fishing strategies (and hence catchability) as different species have been targeted. In the early 1960s, Japanese longliners changed from targeting albacore (Thunnus alalunga) and yellowfin (T. albacares) for the canned-tuna market to bigeye (T. obesus) and yellowfin tuna for the Japanese sashimi market. Japanese longline CPUE for albacore declined rapidly not because of declining albacore abundance, but because of this change in species targeting. By contrast, Taiwanese longliners have consistently targeted albacore in subequatorial waters of all oceans, and their CPUE provides a better index of albacore abundance. These results show that CPUE has declined by 50% over 40 years in the South Pacific, but they do not replicate the rapid and much larger decline in CPUE in the 1960s evident in the Japanese data (Fig.1a). The Myers and Worm analysis excludes data from the equatorial Pacific, where the highest catches are taken and which is the core habitat for tropical tunas. When these data are included, yellowfin-tuna CPUE in the western Pacific is seen to decline by 70% over 50 years, during which time annual catches by longline and other methods increase from insignificant levels in the early 1950s to more than 400,000 tonnes by the late 1990s (Fig. 1b). By contrast, the CPUE for bigeye tuna has been stable for over 40 years, despite continuously increasing catch (Fig. 1c). Changes in fishing strategies designed to target the deeper-swimming and higher-value bigeye tuna occurred during the 1970s (ref. 3), making it unlikely that CPUE accurately reflects changes in abundance for either species unless it is adjusted to account for the shift in targeting. Unadjusted Japanese longline CPUE tends to overestimate abundance decline for yellowfin tuna and underestimate abundance decline for bigeye tuna. Stock assessments rely on a range of data in addition to CPUE, including catch, size composition, tagging and biological data. When stock-assessment models 6 that consider all the available data are applied to Pacific tunas, fishery-induced declines in abundance during the 1950s and 1960s of the magnitude proposed by Myers and Worm are found to be extremely unlikely. Moreover, where declines do occur, they are not, as claimed by Myers and Worm, due exclusively to fishing. It is impossible, for example, under conventional populationdynamics theory to attribute the pre-1970 decline in yellowfin CPUE to fishing at a time when the total catches were less than one-tenth of today’s catches. In summary, the trends in catches and CPUE (Fig. 1) and the results of stock-assessment modelling show that the basic assumption of Myers and Worm that CPUE is proportional to brief communications arising

Use of otolith weight in length-mediated estimation of proportions at age

Each year almost a million fish are aged from otoliths, primarily to estimate proportions at age for use in stock assessments. The preparation and reading of otoliths is time-consuming and thus expensive. Two techniques have been proposed to reduce costs. The first is length-mediated estimation, in which the length distribution from a large sample of fish is converted to an age distribution, using information (usually in the form of an age-length key) from a smaller sample containing length and age data. The second is to infer age from otolith weight (and/or other otolith measurements). These two cost-saving ideas are combined in a new method, length-mediated mixture analysis. It requires three samples - one with lengths only, one with lengths and otolith measurements, and one with lengths, otolith measurements and ages - and estimation is by maximum likelihood. The use of this method, which can be thought of as a generalisation of three established methods of age inference, is illustrated in two simulation experiments in a cost-benefit framework.

Reproductive schedules in southern bluefin tuna: are current assumptions appropriate?

Southern bluefin tuna (SBT) appear to comprise a single stock that is assumed to be both mixed across its distribution and having reproductive adults that are obligate, annual spawners. The putative annual migration cycle of mature SBT consists of dispersed foraging at temperate latitudes with migration to a single spawning ground in the tropical eastern Indian Ocean. Spawning migrations have been assumed to target two peaks in spawning activity; one in September-October and a second in February-March. SBT of sizes comparable to that of individuals observed on the spawning ground were satellite tagged in the Tasman Sea region (2003–2008) and demonstrated both migrations to the spawning grounds and residency in the Tasman Sea region throughout the whole year. All individuals undertaking apparent spawning migrations timed their movements to coincide with the second recognised spawning peak or even later. These observations suggest that SBT may demonstrate substantial flexibility in the scheduling of reproductive events and may even not spawn annually as currently assumed. Further, the population on the spawning grounds may be temporally structured in association with foraging regions. These findings provide new perspectives on bluefin population and spatial dynamics and warrant further investigation and consideration of reproductive schedules in this species.