Range contraction in large pelagic predators

Abstract

Large reductions in the abundance of exploited land predators have led to significant range contractions for those species. This pattern can be formalized as the range-abundance relationship, a general macroecological pattern that has important implications for the conservation of threatened species. Here we ask whether similar responses may have occurred in highly mobile pelagic predators, specifically 13 species of tuna and billfish. We analyzed two multidecadal global data sets on the spatial distribution of catches and fishing effort targeting these species and compared these with available abundance time series from stock assessments. We calculated the effort needed to reliably detect the presence of a species and then computed observed range sizes in each decade from 1960 to 2000. Results suggest significant range contractions in 9 of the 13 species considered here (between 2% and 46% loss of observed range) and significant range expansions in two species (11-29% increase). Species that have undergone the largest declines in abundance and are of particular conservation concern tended to show the largest range contractions. These include all three species of bluefin tuna and several marlin species. In contrast, skipjack tuna, which may have increased its abundance in the Pacific, has also expanded its range size. These results mirror patterns described for many land predators, despite considerable differences in habitat, mobility, and dispersal, and imply ecological extirpation of heavily exploited species across parts of their range.

Fig. 1.

Mapping species ranges. Shown are raw data for Atlantic bluefin tuna (T. thynnus) and Pacific bluefin tuna (T. orientalis) in the 1960s (Left) and 1990s (Right). Depth-corrected Japanese longlining (A and B) and FAO catch data (C and D): white indicates no effort and gray indicates effort but no catch (Japanese data only); otherwise the colors indicate catch. (E and F) Combined raw data for use in the analysis. Gray indicates no data. Orange indicates Japanese data were used. Blue indicates FAO data were used. Presence is indicated by a white diagonal line, absence is indicated by no line. It can be seen how FAO data fill in gaps in the Japanese data, for example in the Gulf of Mexico, allowing the use of much more complete ranges. Cells with catch but without presences indicate occurrences below a specified presence threshold of five individuals (or five tons caught) per decade.

Fig. 2.

Changes in observed tuna and billfish ranges between the 1960s and 1990s. Cells that are occupied in both decades are indicated in gray, range loss over time is shown in blue, and range expansion in red.

Fig. 3.

Range changes by decade. Shown are the observed range changes (±95% confidence interval) in the (A) Atlantic, (B) Indian, and (C) Pacific Ocean. Range extent relative to the 1960s: green symbols represent 1970s, blue 1980s, black 1990s. For species codes refer to Table 1. Positive values indicate a range expansion, negative a range contraction.

Fig. 4.

Abundance vs. observed range changes. Abundance estimates were derived from stock assessments published by the relevant management bodies and include yellowfin, skipjack, albacore, and Atlantic bluefin tuna, as well as swordfish stocks. Fitted line was derived from a generalized additive model.