Fish Conservation two consecutive years of poor oceanic conditions led to spawning failure and collapse However, even when vast populations of naturally small, short-lived fishes such as sardines, anchovies, and herrings collapsed to one-thousandth of their former numbers, high enough numbers have remained to maintain genetic diversity Yet in some animal populations, older or larger individuals are more likely to carry more diversity in the form of heterozygosity – differing forms of particular genes Larger individuals often spend more time on breeding areas than younger ones, so fisheries targeting spawning groups, such as those for groupers, orange roughy, and many others, can subject the most genetically diverse individuals to the most intense mortality This kind of genetic diversity loss is believed by some to be a common consequence of heavy exploitation, even without reducing populations to near-extinction levels Theory suggests that genetic change from fishing is likely to be common, and several studies have found convincing evidence Perhaps the species best-studied before and after commercial exploitation is a long-lived fish called the orange roughy Off New Zealand, only years of heavy commercial exploitation reduced populations by 70% and significantly reduced genetic diversity within those populations, probably because older individuals in this very long-lived species were more heterozygous In one North Sea cod population, intensive fishing reduced the chances of a young cod surviving to breeding age by 95% Over several decades, average age of sexual maturation declined from about 10 to 7–8 years, apparently through genetic change Some populations of flounder and haddock also show convincing evidence of genetic change In a population of Atlantic salmon, average age at maturity had dropped after two decades of heavy fishing In a study of chinook salmon, average spawning age fell by years Other studies on salmon showed other changes In sum, some studies found no apparent changes attributable to fishing, some showed changes that were inconclusively genetic, and other studies showed genetic changes that were driven by intense fishing Species Diversity Human activities significantly change species composition and abundance, and predator–prey and competitive relationships Naturally evolved numerical and functional relationships among species are sometimes referred to as the community’s ecological integrity One indication that many regions’ ecological integrity has greatly diminished is the changing world catch Since 1950, many fisheries, forced to work lower on the food chain as they deplete large fish, have shown a gradual transition from catching mostly large, long-lived, fish-eating bottomfish such as cod toward catching small, plankton-eating, open-ocean fishes like herring, as well as short-lived, low-on-the-foodchain invertebrates like squid (Pauly et al., 1998) Daniel Pauly, the eminent ecologist who helped discover this trend, remarked, ‘‘If things go unchecked, we might end up with a marine junkyard dominated by plankton.’’ Various marine communities have been changed by overfishing A common pattern – particularly acute in coral reef systems but also seen in kelp communities and elsewhere – is 451 selective removal of the largest species first, then of large individuals of smaller species People removed most large groupers and sea basses from many coral reefs and kelp forests, and manatees, dugongs, and sea turtles from seagrass communities, before scientists ever studied these habitats Though no one really understands how those communities are evolved to function, they were certainly very different Since Columbus landed in the Bahamas, for instance, Caribbean sea turtle populations have declined 99% (Jackson, 1997) Depletion of fish populations changes patterns of abundance, distribution, and competition among plants, urchins, corals, sponges, tunicates, and other creatures Alarming declines in North Pacific seabirds, Steller’s sea lions, and other wildlife may be caused by heavy fishing When fishing depleted Barents Sea herring and capelin, cod failed for lack of food Squid sometimes increase following the collapse of their overfished predators Off New England and maritime Canada, overfishing shifted communities dominated by cod, haddock, and flounder to domination by spiny dogfish (a small shark) and skates (a type of ray) But not even skates are safe Fishing has driven the commonest skate in the northeast Atlantic, Raja batis, to local extinction in the Irish Sea Vulnerability of Marine Fishes to Extinction Few fully marine creatures, and perhaps no fully marine fish species, have gone extinct because of human activities Many marine fishes have large distributions and a greater chance for replacement by neighboring populations if they are locally wiped out But the important thing is to prevent, not document, endangerment and extinctions The time to save a species is when it is still common Waiting until biodiversity declines, until populations falter, and until more species names get added to various endangered lists will only ensure three things: (1) more species will get into serious trouble, (2) the value to humanity from extremely important natural resources such as fishes will be seriously diminished, creating more of the economic, social, and nutritional problems already suffered by people in too many communities throughout the world, and (3) fixing the problems will be much harder, more expensive, and prone to failure Numerous fish have already been listed on the U.S Endangered Species List, on the U.S Overfished Species List, and under the world Convention on International Trade in Endangered Species (CITES) and the World Conservation Union’s Red List The Red List notes that hundreds of marine fish have suffered major and rapid population declines or significant local extinctions (extirpations) Most vulnerable are those that breed in freshwater but mature in salt water – the anadromous species – such as salmon and sturgeons, because they suffer overfishing plus the effects of habitat disruption in rivers that are heavily dammed, subject to intensive agricultural water withdrawals, or subject to massive siltation from destructive logging practices Hundreds of North American salmon runs are already extinct, and the trends for sturgeons and totoaba (a large fish of the croaker family that is dependent on Colorado River flow into the Gulf of California) suggest that biological extinction may become a real possibility Groupers are also vulnerable; they are generally slowgrowing fishes whose spawning groups are easily targeted by