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Overfishing i.e. catch exceeds replacement – changes in genetic diversity • harvesting larger specimens selective pressure for smaller animals • harvesting early in reproductive cycle selective pressure for fast maturation • surviving individuals no longer represent full genetic variability (genetic drift) increased risk of extinction Northern Elephant Seal Overfishing (continued) – changes/loss in species diversity • Extirpated (locally extinct) species cannot fulfill their ecological functions potential impact on ecosystem diversity Controlling Overfishing – Exclusive economic zones (EEZs) coastal nations control fishing within 200 miles counter-acting “Tragedy of the Commons” Controlling Overfishing (cont.) - New fisheries e.g., Alaskan Pollock (surimi) or krill as a consequence of decreasing traditional fisheries (e.g., tuna; great whales) Antarctic krill Surimi Controlling Overfishing (cont.) – Consumer education hoping to curb demand for endangered species e.g. www.oceanwise.ca Other factors affecting marine fisheries – destruction/development of coastal habitats loss of feeding, breeding and nursery grounds for commercial fishes – wasteful and destructive fishing practices • inefficient use of the catch (e.g. filleting, shark finning!!) • incidental catch (a.k.a. bycatch or “trash fish”) • habitat destruction (sea floor) Trawling produces large by-catch and destroys benthic habitat Drift nets produce large by-catch and lost nets ghost fish Other factors affecting marine fisheries – aquaculture: the use of agricultural techniques to breed and raise marine organisms Aquaculture (cont.) • monoculture: only 1 species is raised • polyculture: several species are raised together • fish aquaculture: net cages vs. pens • raft culture: juveniles of commercially valuable molluscs (clams, mussels, oysters) are collected and attached to ropes suspended from rafts • shrimp farming • eco-friendly aquaculture Aquaculture (cont.) • Problems – mangrove ecosystems are destroyed in Ecuador and Asia to make room for shrimp farms which quickly become polluted from accumulated wastes and abandoned – large numbers of fish must be caught to supply food for shrimp and salmon aquaculture, making these fish unavailable to support many fish species in the wild – antibiotics and pesticides along with nutrients used in aquaculture become harmful runoff into coastal waters
