Global population trajectories of tunas and their relatives

Abstract

Tunas and their relatives dominate the world's largest ecosystems and sustain some of the most valuable fisheries. The impacts of fishing on these species have been debated intensively over the past decade, giving rise to divergent views on the scale and extent of the impacts of fisheries on pelagic ecosystems. We use all available age-structured stock assessments to evaluate the adult biomass trajectories and exploitation status of 26 populations of tunas and their relatives (17 tunas, 5 mackerels, and 4 Spanish mackerels) from 1954 to 2006. Overall, populations have declined, on average, by 60% over the past half century, but the decline in the total adult biomass is lower (52%), driven by a few abundant populations. The trajectories of individual populations depend on the interaction between life histories, ecology, and fishing pressure. The steepest declines are exhibited by two distinct groups: the largest, longest lived, highest value temperate tunas and the smaller, short-lived mackerels, both with most of their populations being overexploited. The remaining populations, mostly tropical tunas, have been fished down to approximately maximum sustainable yield levels, preventing further expansion of catches in these fisheries. Fishing mortality has increased steadily to the point where around 12.5% of the tunas and their relatives are caught each year globally. Overcapacity of these fisheries is jeopardizing their long-term sustainability. To guarantee higher catches, stabilize profits, and reduce collateral impacts on marine ecosystems requires the rebuilding of overexploited populations and stricter management measures to reduce overcapacity and regulate threatening trade.

Fig. 1.

Age-structured stock assessments were available for 26 populations (11 species) of tunas and their relatives. (A) Geographic locations. (B) Temporal span. Abbreviations for population names: E., east; G.O.M., Gulf of Mexico; N., north; N.E., northeast; S., south; T.C., Tsushima Current; U.S., United States; W., west.

Fig. 2.

Global catches and adult biomass trajectories of tunas and their relatives. (A) Catches of the major taxonomic groups of tunas and their relatives in the world from 1950 through 2008. (B) Relative adult biomass summed across 26 populations of tunas and their relatives (thick solid line), standardized to 1 in 1954. Faint gray lines and black dashed lines show the effect of excluding a single population at a time from the global trend and recalculating the relative adult biomass. The dashed line shows the effect of excluding the most influential population. (C) Estimated overall extent of decline in total adult biomass from 1954 to 2006 (filled diamond) and the effect of excluding a single population at a time and recalculating the total extent of decline (shaded circles).

Fig. 3.

Adult biomass trajectories of tunas and their relatives within oceans (A–D), taxonomic and ecological groups (E–H and L), and life histories (I–L). (A, E, and I) Total adult biomass in million tonnes. (B–D, F–H, and J–L) Relative adult biomass across all populations (thick solid line) standardized to 1 in 1954. Faint gray and dashed lines show the effect of excluding a single population at a time and recalculating the relative adult biomass. Dashed lines show the effect of excluding the most influential populations. The adult biomass of Spanish mackerels was eliminated from E because their absolute adult biomass was negligible relative to the other groups. Albacore tuna and the Atlantic, Pacific, and Southern bluefin tunas are considered temperate tunas, and skipjack, yellowfin, and bigeye tunas are considered tropical tunas. The maximum body size of a particular species is provided in Table S1. Abbreviations for population names: G.O.M., Gulf of Mexico; N., north; N.E., northeast; W., west.

Fig. 4.

Current exploitation status and fishing mortality rate over time of tunas and their relatives. (A) Reference points for tunas and their relatives: current adult biomass relative to B_MSY (x axis) vs. current exploitation rate relative to F_MSY (y axis). Codes follow Fig. 1 and Table S2. Colors represent the kernel density of the points. (B) Fishing mortality rate over time across tunas and their relatives. Faint gray lines and black dashed lines show the effect of excluding a single population at a time and recalculating the overall fishing mortality rate. Dashed lines represent the most influential populations. E., east; N.E., northeast; Pac., Pacific; S., south; W., west.