29. Read the text and think over the best title for each paragraph: Predator fish

1. Medium size pelagic fishes include trevally, barracuda, flying fish, bonito, mahi mahi and coastal mackerel. Many of these fish hunt forage fish, but are in turn hunted by yet larger pelagic fish. Nearly all fish are predator fish to some measure, and apart from the top predators, the distinction between predator fish and prey or forage fish is somewhat artificial.

2. Around Europe there are three populations of coastal mackerel. One population migrates to the North Sea, another stays in of the Irish Sea, and the third population migrates southwards along the west coast of Scotland and Ireland. The mackerel's cruise speed is an impressive 10 kilometres per hour.

3. Many large pelagic fish are oceanic nomadic species which undertake long offshore migrations. They feed on small pelagic forage fish, as well as medium sized pelagic fish. At times, they follow their schooling prey, and many species form schools themselves. Examples of larger pelagic fish are tuna, billfish, king mackerel and sharks and large rays.

4. Tuna in particular are of major importance to commercial fisheries. Though tuna migrate across oceans, trying to find them there is not the usual approach. Tuna tend to congregate in areas where food is abundant, along the boundaries of currents, around islands, near seamounts, and in some areas of upwelling along continental slopes. Tuna are captured by several methods: purse seine vessels enclose an entire surface school with special nets, pole and line vessels which use poles baited with other smaller pelagic fish as baitfish, and rafts called fish aggregating devices are set up, because tuna, as well as some other pelagic fish, tend to congregate under floating objects.

5. Other large pelagic fish are premier game fish, particularly marlin and swordfish. Yellowfin tuna are being fished as a replacement for the now largely depleted Southern bluefin tuna.

 

30. Read the text and translate it. Productivity

Upwelling occurs both along coastlines and in midocean when a collision of deep ocean currents brings cold water rich in nutrients to the surface. These upwellings support blooms of pytoplankton, which in turn produce zooplankton and support many of the world's main fisheries. If the upwelling fails then fisheries in the area fail.

In the 1960s the Peruvian anchoveta fishery was the world's largest fishery. The anchoveta population was greatly reduced шт 1972, when warm water drifted over the cold Humboldt Current, as part of a 50 year cycle, lowering the depth of the thermocline. The upwelling stopped and phytoplankton production plummeted, as did the anchoveta population, and millions of seabirds, dependant on the anchoveta, died. Since the mid 1980s, the upwelling has resumed, and the Peruvian anchoveta catch levels have returned to the 1960s levels.

Off Japan, the collision of the Oyashio Current with the Kuroshio Current produces nutrient-rich upwellings. Cyclic changes in these currents resulted in a decline in the sardine sardinops melanosticta populations. Fisheries catches fell from 5 million tonnes in 1988 to 280 thousand tonnes in 1998. As a further consequence, Pacific bluefin tuna stopped moving into the region to feed.

Ocean currents can shape how fish are distributed, both concentrating and dispersing them. Adjacent ocean currents can define distinct, if shifting, boundaries. These boundaries can even be visible, but usually their presence is marked by rapid changes in salinity, temperature and turbitity.

For example, in the Asian northern Pacific, albacore are confined between two current systems. The northern boundary is determined by the cold North Pacific Current and the southern boundary is determined by the North Equatorial Current. To complicate things, their distribution is further modified within the area defined by the two current systems by another current, the Kuroshio Current, whose flows fluctuate seasonally.

Epipelagic fish often spawn in an area where the eggs and larvae drift downstream into suitable feeding areas, and eventually drift into adult feeding areas.

Islands and banks can interact with currents and upwellings in a manner that results in areas of high ocean productivity. Large eddies can form downcurrent or downwind from islands, concentrating plankton. Banks and reefs can intercept deep currents that upwell.