John Hampton

El Niño Southern Oscillation and tuna in the western Pacific

Nearly 70% of the worlds annual tuna harvest, currently 3.2 million tonnes, comes from the Pacific Ocean. Skipjack tuna ( Katsuwonus pelamis ) dominate the catch. Although skipjack are distributed in the surface mixed layer throughout the equatorial and subtropical Pacific, catches are highest in the western equatorial Pacific warm pool, a region characterized by low primary productivity rates that has the warmest surface waters of the worlds oceans (Fig. 1). Assessments of tuna stocks indicate that recent western Pacific skipjack catches approaching one million tonnes annually are sustainable. The warm pool, which is fundamental to the El Niño Southern Oscillation (ENSO) and the Earths climate in general, must therefore also provide a habitat capable of supporting this highly productive tuna population. Here we show that apparent spatial shifts in the skipjack population are linked to large zonal displacements of the warm pool that occur during ENSO events,. This relationship can be used to predict (several months in advance) the region of highest skipjack abundance, within a fishing ground extending over 6,000 km along the Equator.

Biomass, Size, and Trophic Status of Top Predators in the Pacific Ocean

Fisheries have removed at least 50 million tons of tuna and other top-level predators from the Pacific Ocean pelagic ecosystem since 1950, leading to concerns about a catastrophic reduction in population biomass and the collapse of oceanic food chains. We analyzed all available data from Pacific tuna fisheries for 1950–2004 to provide comprehensive estimates of fishery impacts on population biomass and size structure. Current biomass ranges among species from 36 to 91% of the biomass predicted in the absence of fishing, a level consistent with or higher than standard fisheries management targets. Fish larger than 175 centimeters fork length have decreased from 5% to approximately 1% of the total population. The trophic level of the catch has decreased slightly, but there is no detectable decrease in the trophic level of the population. These results indicate substantial, though not catastrophic, impacts of fisheries on these top-level predators and minor impacts on the ecosystem in the Pacific Ocean.

A spatially disaggregated, length-based, age-structured population model of yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean

A spatially disaggregated, length-based, age-structured model for yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean is described. Catch, effort, length-frequency and tagging data stratified by quarter (for the period 1962–99), seven model regions and 16 fisheries are used in the analysis. The model structure includes quarterly recruitment in each region, 20 quarterly age classes, independent growth patterns for juveniles and adults, structural time-series variation in catchability for all non-longline fisheries, age-specific natural mortality, and age-specific movement among the model regions. Acceptable fits to each component data set comprising the log-likelihood function were obtained. The model results suggest that declines in recruitment, and as a consequence, population biomass, have occurred in recent years. Although not obviously related to over-exploitation, the recruitment decline suggests that the productivity of the yellowfin tuna stock may currently be lower than it has been previously. Recent catch levels appear to have been maintained by increases in fishing mortality, possibly related to increased use of fish aggregation devices in the purse-seine fishery. A yield analysis indicates that average catches over the past three years may have slightly exceeded the maximum sustainable yield. The model results also reveal strong regional differences in the impact of fishing. Such heterogeneity in the fisheries and the impacts on them will need to be considered when future management measures are designed.

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

Mobility of tropical tunas and the implications for fisheries management

We apply an advection-diffusion reaction model to data from three different tuna tagging experiments in the western and central Pacific Ocean (WCPO) to reexamine the question of to what extent the population dynamics and spatial characteristics of tropical tunas require international cooperation for effective management. The median lifetime displacement of skipjack ranges from 420 to 470 nautical miles. The lifetime displacement of yellowfin is about 20% less. The median half-life, a measure of residence time, of skipjack and yellowfin in WCPO exclusive economic zones (EEZs) is 3-6 months. Fishing decreases the lifetime displacement and decreases the half-life. We conclude that international arrangements between neighboring EEZs are essential for effective conservation, but that Pacific Island countries can achieve benefits from domestic conservation and fishery development policies.

Modelling the impact of climate change on Pacific skipjack tuna population and fisheries

IPCC-type climate models have produced simulations of the oceanic environment that can be used to drive models of upper trophic levels to explore the impact of climate change on marine resources. We use the Spatial Ecosystem And Population Dynamics Model (SEAPODYM) to investigate the potential impact of Climate change under IPCC A2 scenario on Pacific skipjack tuna (Katsuwonus pelamis). IPCC-type models are still coarse in resolution and can produce significant anomalies, e.g., in water temperature. These limitations have direct and strong effects when modeling the dynamics of marine species. Therefore, parameter estimation experiments based on assimilation of historical fishing data are necessary to calibrate the model to these conditions before exploring the future scenarios. A new simulation based on corrected temperature fields of the A2 simulation from one climate model (IPSL-CM4) is presented. The corrected fields led to a new parameterization close to the one achieved with more realistic environment from an ocean reanalysis and satellite-derived primary production. Projected changes in skipjack population under simple fishing effort scenarios are presented. The skipjack catch and biomass is predicted to slightly increase in the Western Central Pacific Ocean until 2050 then the biomass stabilizes and starts to decrease after 2060 while the catch reaches a plateau. Both feeding and spawning habitat become progressively more favourable in the eastern Pacific Ocean and also extend to higher latitudes, while the western equatorial warm pool is predicted to become less favorable for skipjack spawning.

Ecological effects of longline fishing and climate change on the pelagic ecosystem off eastern Australia

Pelagic longline fisheries target (or catch incidently) large apex predators in the open ocean (e.g. tunas, billfish and sharks) and have the potential to disrupt the ecosystem functionality if these predators exert strong top–down control. In contrast, warming of oceans from climate change may increase bottom–up effects from increases in primary productivity. An ecosystem model of a large pelagic ecosystem off eastern Australia was constructed to explore the potential ecological effects of climate change and longlining by Australia’s Eastern Tuna and Billfish Fishery. The model reproduced historic biomass and fishery catch trends from 1952 to 2006 for seven functional groups. Simulated changes in fishing effort and fishing mortality rate on individual target species from 2008 to 2018 resulted in only modest (<20%) changes in the biomass of target species and their direct predators or competitors. A simulated increase in phytoplankton biomass due to climate change resulted in only small increases (<11%) in the biomass of all groups. However, climate-related changes to the biomass of micronekton fish (−20%) and cephalopods (+50%) resulted in trophic cascades. Our results suggest there may be ecological redundancy among high trophic level predators since they share a diverse suite of prey and collectively only represent <1% of the total system biomass. In contrast, micronekton fishes and cephalopods have high biomasses and high production and consumption rates and are important as both prey and predators. They appear to exert ‘wasp–waist’ control of the ecosystem rather than top–down or bottom–up processes reported to drive other pelagic systems.

Maximizing Resource Rent From the Western and Central Pacific Tuna Fisheries

Rent generated by the tuna fisheries occurring in the waters of Pacific Islands Nations is estimated for various levels and combinations of purse-seine, pole-and-line, frozen tuna longline, and fresh tuna longline fishing effort, using a multi-species, multi-fleet bioeconomic model. The underlying population model integrates available information on the population dynamics of skipjack, yellowfin, bigeye, and Southern albacore tunas in the Pacific Ocean. The economic model utilizes the most recent data on fishing effort costs for the purse seine, pole-and-line, and longline fleets operating in the western and central Pacific Ocean, along with recent estimates of prices by species, method of capture and market, and estimates of demand elasticities. The results of the model indicate that fishery rent could be increased substantially above the current level by decreasing the size of all fleets, with the possible exception of the tuna longline fleet. The results also suggest that the countries of the region could benefit significantly by changing the level and structure of access fees levied as a percentage of total catch revenue.

Shifting from marine reserves to maritime zoning for conservation of Pacific bigeye tuna (Thunnus obesus)

Over 50% of the total bigeye tuna (BET) landed in the Western Central Pacific Ocean is caught incidentally in the purse seine fishery and sold for canning at prices less than US

An ocean observation system for monitoring the affects of climate change on the ecology and sustainability of pelagic fisheries in the Pacific Ocean

2/kg. The remainder is landed in longline fisheries directed at BET and sold as fresh or frozen tuna at prices greater than US