8. ECOSYSTEM COMPARISONS
Changes in the Clyde Sea ecosystem are similar in many aspects to those which have occurred in marine ecosystems the world over as a result of natural and anthropogenic impacts. Each source of impact may induce different patterns of response in marine ecosystems that reflect the different biological production characteristics of the ecosystem. In addition, coastal, shelf and marginal sea ecosystems which are heavily subjected to human induced perturbations are thought to experience more complicated regime changes under multiple forcing than oceanic ecosystems (Oguz and Gilbert, 2007). Understanding the structure and function of marine ecosystems and their response to such perturbations can be aided by a comparative approach (Megrey et al., 2009).
The Black Sea is a marine ecosystem which has also experienced a collapse in commercial fisheries and has undergone dramatic changes in its ecological properties from the early 1970's - 1990's. The Black Sea, although on a larger scale, is similar to the Clyde in that it is a semi-enclosed ecosystem with a history of intense commercial fishing pressure and heavy organic pollution. The two ecosystems also share similar trophic characteristics which at one point supported stocks of small pelagic fish.
The Black Sea receives extraordinarily high nutrient loading and contaminants from rivers that drain half of Europe and parts of Asia (Oguz et al., 2001). Uncontrolled, industrial fisheries for pelagic species (bonito, mackerel, bluefish, and dolphins) started on a huge scale in the early 1970s and within 20 years the stocks became severely depleted (Daskalov, 2002). A study by Oguz and Gilbert (2007) illustrated the existence of regime-shift events, from ecological data (1960-2000), due to the synchronous forcing of marked nutrient enrichment, overfishing and climatic cooling/warming. The first major change in the food web of the Black Sea was the decline of large predator fish populations and the subsequent increase of small pelagic fish stocks. The second shift corresponded to the collapse of the small pelagic stocks followed by an immediate rise of gelatinous stocks in the late 1980s. It was concluded by Oguz and Gilbert that the regime shifts in the Black Sea supports strongly the assertion that the combination of eutrophication and extreme overfishing can induce hysteresis in marine ecosystems if they exert sufficiently strong forcing on the system.
Heath and Speirs (2010) have suggested a similar hysteresis has occurred in the Clyde ecosystem where there has been a transition to an alternative stable state dominated by small fish (see Section 7).
Northwest Atlantic ecosystems have historically supported the largest and densest cod stocks. However, many of these cod stocks have steeply declined (Hamilton et al., 2004; Christensen et al., 2003; Myers et al., 2001). Biological analyses point to overfishing as the primary cause of the collapse with environmental conditions being a contributing factor (Hutchings and Myers, 1993; Drinkwater 2002). In the Gulf of St Lawrence a fishing moratorium was put in place by the Canadian government in 1992 to promote the recovery of these stocks. Despite this moratorium on Northern cod in 1992 and subsequent strict management efforts the stock has failed to rebound and in April 2003 the Northern cod fishery was closed (Hamilton et al., 2004). Similarly the cod stocks to the west of Scotland have not shown any significant signs of a rebound despite the strict management strategies in place to protect cod.
Similarly to the Clyde fisheries, since the collapse of cod and other groundfish species in the Gulf of St. Lawrence, fishing effort has been more directed towards shellfish fisheries (Morissette et al., 2007). As cod catches declined in the early 1970s and 1980s effort was redirected towards invertebrate fisheries, most notably northern shrimp ( Pandulus borealis) and American lobster ( Homarus americanus). Much like the Clyde, the total value of the new invertebrate fisheries exceeded that of cod when catches were at their maximum (Hamilton et al., 2004).
The Mediterranean fishery has experienced a decline in catches over the past 20 years due to an excessive increase in effort (Sarda et al., 2004). Hake is the most important target species of the mixed demersal fishery in the western Mediterranean and landings have been decreasing from a maximum since the early 1990s. In addition, an increasing share of the catch has been immature individuals (Sarda et al., 2005). Discarding is a major problem, similar to the Clyde, and in the Mediterranean small juvenile hake are the hardest hit. As a result, a lot of research effort has been invested to improve selectivity and reduce by-catch levels (Cheret et al., 2002; Alemany and Alvarez, 2003; Sarda et al., 2004; Sarda et al., 2005).
Studies and simulations of escape systems for Mediterranean hake suggest that allowing the escape of small fish could bring about a 30-50% increase in catches of larger individuals four or five years later after implementation of the system (Cheret et al., 2002). Selectivity studies for the hake fishery have shown that in general the use of sorting grids results in high escape rates for individuals that are immature or under minimum landing size (Sarda et al., 2004). The use of a square mesh cod-end in this fishery has also been shown to allow more fish to escape but is much less efficient than the sorting grid (Sarda et al., 2004).
Recently the General Fisheries Commission for the Mediterranean ( GFCM) incited the Mediterranean countries to substitute the diamond meshes of the trawl cod-end by 40 mm square meshes for the bottom trawling fleet ( GFCM, 2011). In addition, GCFM has encouraged the Mediterranean countries to develop more selectivity studies in various situations and to include bio economical analysis of their use. This has been a significant step towards the implementation of sorting grids in commercial trawl gears in the demersal fisheries of the Mediterranean Sea.