Extinction, survival or recovery of large predatory fishes

Abstract

Large predatory fishes have long played an important role in marine ecosystems and fisheries. Overexploitation, however, is gradually diminishing this role. Recent estimates indicate that exploitation has depleted large predatory fish communities worldwide by at least 90% over the past 50-100 years. We demonstrate that these declines are general, independent of methodology, and even higher for sensitive species such as sharks. We also attempt to predict the future prospects of large predatory fishes. (i) An analysis of maximum reproductive rates predicts the collapse and extinction of sensitive species under current levels of fishing mortality. Sensitive species occur in marine habitats worldwide and have to be considered in most management situations. (ii) We show that to ensure the survival of sensitive species in the northwest Atlantic fishing mortality has to be reduced by 40-80%. (iii) We show that rapid recovery of community biomass and diversity usually occurs when fishing mortality is reduced. However, recovery is more variable for single species, often because of the influence of species interactions. We conclude that management of multi-species fisheries needs to be tailored to the most sensitive, rather than the more robust species. This requires reductions in fishing effort, reduction in bycatch mortality and protection of key areas to initiate recovery of severely depleted communities.

Figure 1

The present status of large predatory fishes. (a) Estimates of the residual biomass proportion for predatory fish communities on regional and global scales. (b) Estimates of residual spawning stock biomass proportion for Atlantic cod (Gadus morhua) populations. Error bars are 95% confidence intervals (where available), except for large fishes (4–16 and 16–66kg), where bars refer to the range of possible outcomes under different assumptions about ecological transfer efficiencies. In (a) the time-scale of the study and species groups are indicated. Estimates were derived from the following sources: Friedlander & DeMartini (2002), Hawaiian reefs; Jennings & Blanchard (2004), North Sea; Christensen et al. (2003), North Atlantic; Myers & Worm (2003), global; Ward & Myers (2003), north Pacific; Tang et al. (2003), Bohai Sea; Baum & Myers (2004), Gulf of Mexico; Vacchi et al. (2000), Mediterranean Sea; Baum et al. (2003), northwest Atlantic. Estimates in (b) were derived from an analysis of cod carrying capacity (closed circles: Myers et al. 2001b) and cod recruitment (open circles: see text (§ 2) for details). The shaded bar refers to the proportional biomass commonly assumed to allow for maximum sustainable yield (0.3 < B_MSY < 0.5).

Figure 2

Predicting the occurrence of sensitive species. (a) The lifetime maximum reproductive rate $\widehat{\alpha }$, (b) the annual maximum reproductive rate $\tilde{\alpha }$, and (c) the predicted extinction fishing mortality F_extinct are shown in relation to the mean temperature of the habitat. Fishing mortality was converted from an instantaneous rate to the proportion removed per year, and assumed zero to the midpoint between age at recruitment and age at maturity, and constant thereafter.

Figure 3

Predicting extinctions of sensitive species. Shown is the fishing mortality required to drive populations of sharks (dotted line) and bony fishes (solid line) to extinction under three scenarios. The scenarios are (a) fishing mortality is constant at the age of recruitment, (b) fishing mortality is constant at the midpoint between age at recruitment and age at maturity, and (c) fishing mortality is constant at the age of first maturity. Lines represent the cumulative probability of extinction obtained by smoothing across individual estimates for 26 species of sharks and 151 species of bony fishes, respectively. In each case fishing mortality is assumed zero to a given age, and constant thereafter. Fishing mortality was converted from an instantaneous rate to the proportion removed per year for clarity.

Figure 4

Predicting survival of sensitive species. The minimum proportional reduction of instantaneous rate of fishing mortality needed to allow the survival of shark species in the northwest Atlantic was calculated under two assumptions: that juvenile mortality is the same as adult mortality at low population sizes (filled circles) and that juvenile mortality for the first year is twice the adult mortality at low population sizes (open circles). See text (§ 4) for details.

Figure 5

Predicting community recovery. Data compiled from 85 no-take marine reserves demonstrate rapid increases in (a) biomass and (b) diversity of fish communities, when fishing mortality is reduced to zero. Data from Halpern (2003). Positive values indicate increases, negative values decreases in biomass or diversity within a reserve. A log-ratio of 0.3 equals a 100% increase.