188.8.131.52 Predicting future yields of commercial and forage stocks
Quantitative predictions of the responses of commercial and forage fish stocks to changes in ocean temperature are very difficult to make because (a) resolution of existing predictive models is insufficient, (b) exploitation has already altered stock sizes and basic biology/ecology, so stocks cannot be expected to react as they did historically, and (c) the effect of a moderate climate change (+1-3°C) may well be of less importance than sound fisheries policies and their effective enforcement. The examples given in Box 15.1 show how such climate change, exploitation and other factors affecting marine ecosystems may interact strongly at the northern extreme of the range of a species, while having less effect on the same species further south (e.g., Rose et al., 2000; Rose, 2004; Drinkwater, 2005).
Box 15.1. Atlantic cod in the 20th century: historical examples
This box illustrates how cod populations have responded through the 20th century and until 2005 to the multiple stresses of ocean temperature change, exploitation, changing abundance of food species and predators. All four cod populations show substantial decreases to the present, and suggest that they are now vulnerable to future changes in both climate and exploitation. However, the data suggest that the vulnerability of stocks south of the polar front is less than that to the north of the polar front.
Ocean temperature records, begun off West Greenland in the 1870s, showed very cold conditions until temperatures warmed suddenly around 1920, and maintained high levels until they dropped suddenly in the late 1960s (Jensen, 1939; Buch et al., 1994; Vilhjálmsson, 1997; see Figure 15.5a). There were no Atlantic cod in Greenland waters in the latter half of the 19th century (Jensen, 1926; Buch et al., 1994), while there was a good cod fishery off Iceland (Jensen, 1926, 1939; Buch et al., 1994). Concurrent with the warming in the early 1920s, large numbers of juvenile cod drifted from Iceland to West Greenland and started a self-supporting stock there, which vanished in the 1970s (Buch et al., 1994; Vilhjálmsson, 1997; Vilhjálmsson et al., 2005).
Figure 15.5. The geographical distribution of four major cod stocks in the North Atlantic (red patches). The continuous blue line indicates an average geographical position of the Polar Front. The graphs (a: West Greenland; b: Newfoundland/Labrador; c: Barents Sea, and d: Iceland) show the developments of fishable stock (yellow shading), catches (red line) and temperature (blue line) during the period 1900-2005. Data sources: Greenland (Buch et al., 1994; Horsted, 2000; ICES, 2006); Newfoundland/Labrador (Harris, 1990; Lilly et al., 2003); Iceland (Schopka, 1994; Hafrannsóknastofnunin, 2006; ICES, 2006); Barents Sea (Bochkov, 1982; Hylen, 2002; ICES, 2005a), data since 1981 were kindly provided by the Polar Research Institute of Marine Fisheries and Oceanography (PINRO), Murmansk, Russia.
Comparison of catches and temperature records shows that the occurrence of cod off Greenland depends principally on West Greenland water temperature (Horsted, 2000). Thus the reappearance of cod off Greenland will probably depend on drift of juvenile cod from Iceland as it did in the 1920s. West Greenland waters may now be sufficiently warm for this to happen. Indeed such drifts did occur in 2003 and 2005, but the numbers were small; a probable consequence of the depleted and younger spawning stock of Icelandic cod, which has not produced a strong year class for 20 years (ICES, 2005b).
Beginning in the 16th century, annual catches from this stock increased until the mid-1800s. From 1920 to 1960, catches varied then increased rapidly, peaking in 1968. Catches then dropped sharply until about 1977 when Canada acquired its 200-mile Exclusive Economic Zone (Rose, 2004). Total allowable catches were increased in the mid- to late 1980s, but dropped again after 1989. A moratorium on fishing was imposed in 1992 (see Figure 15.5b). While fishing was the primary cause of the decline (Walters and Maguire, 1996), decreased productivity in cod and a reduction in their primary food (capelin) also occurred in the late 1980s and 1990s (Rose and O’Driscoll, 2002; Shelton et al., 2006). Furthermore, a moratorium on seal hunting led to an explosion in seal abundance and thus increased mortalities of their prey, including cod (Lilly et al., 2003). High mortality remains the key obstacle to population growth. While future warming climate is likely to promote recovery of cod (Drinkwater, 2005), an increase in abundance of the main forage fish, capelin, is likely to be a necessary precursor.
In the face of exploitation, the comparative resilience of these cod stocks is high because they are south of the polar front and, therefore, well inside the limit tolerance of the species to cold temperatures (see Figure 15.5c, d).
Changes in distribution of commercial stocks may lead to conflicts over fishing rights and will require effective negotiations to generate solutions regarding international co-operation in fisheries management (Vilhjálmsson et al., 2005).
In addition, rising water temperatures will lead to an increased risk of harmful algal blooms and occurrences of other marine pests and pollution, hazards that will be multiplied by increased shipping (Loeng et al., 2005; Vilhjálmsson et al., 2005).