top of page

Not so Many Fish in the Sea

For many centuries fish have provided the human population with a large source of protein. Huxley (1883) convinced the public and fellow scientists that fish stocks could never be over-exploited because there are so many fish and they reproduce so quickly. However this was obviously incorrect as 53% of world fisheries are fully exploited and 32% are over exploited. Over exploitation is perhaps the biggest issue of fisheries on the marine ecosystems, however fishing has also lead to direct disturbance to benthic communities, indirect effects leading to ecosystem shifts and a change in the life histories of many species.

Global fish landings rose rapidly after the industrial revolution as technologies and machinery improved allowing fishers to catch more fish with less effort and in new areas. Fish takes rose until the early 1990’s where the increase levelled off at around 80million tonnes a year, with a first sale value of $50 billion. There is a further suspected 11-26 tonnes court annually illegally. The increase in fish landings stopped due to the fact that there just isn’t enough fish left.

As human population rose after the industrial revolution more people needed fish and also there was a huge demand for cheap fish produce to feed terrestrial livestock. This put huge pressure on fish stocks, especially 5 species who contribute to 15% of all global landings (anchovy, Pollock, jack mackerel, herring and mackerel). This pressure has led so far to 14% of the world’s major fish stocks collapsing.

Perhaps the most notable of these collapses is the collapse of the Canadian cod fishery in 1992. The cod population went commercially extinct at this time. The population at this point was thought to be around 22000 T compared to 1.6 million T in 1962. The collapse of fisheries have great economic consequences due to the loss of business, still today this cod stock shows little sign of recovery.

Fish stocks collapse when the fishing capacity of the fleets outweighs the biological productivity of the stock. Catch rates must never exceed the rates of biological replenishment. There is an optimum stock size where the intrinsic rate of growth is high so the new recruits can be harvested leaving a sustainable population size behind.

Once populations reach a certain size then there could be no coming back. A meta-analysis of 90 fish stocks showed that only 7% were able to recover from over-exploitation. A possible explanation for this is the allee effect. This is where there is a positive relationship between population size and fitness, meaning at low populations there could be negative growth due to reduced conspecific interactions.

An example of this effect is the collapse of the queen conch fishery in Florida. The fishery has been closed for 15 years due to overexploitation and still populations are not high. This is because mating doesn’t occur in densities of below 56 conch per hectare due to sperm distribution restrictions. It’s not just reduced mating opportunity, but many fish species rely on the dilution effect when it comes to predators (safety in numbers) so if numbers are reduced there is more risk of predation.

It is often fish with slow growth, late maturity and large body size that get caught. Because of these life history traits it makes it very hard for a population to bounce back as they are not given enough time to reach sexual maturity. This is the case for the Stella sea cow, this large slow moving relation of the manatee went extinct due to over fishing in 1768.

The vastly improved fleets don’t just destroy populations of target species but also that of other species. 27% of all catches are discarded as they contain by-catch which is non target species or juveniles of the target species. The discard is simply thrown back into the sea. Many species are effected including whales, dolphins, shark, birds and turtles.

The Bluefin tuna fleet deploys 107 million baited hooks annually which killed 44000 albatross prior to 1989. For shrimp trawling in north Australia the weight of by catch is in a ratio of 5:1 with weight of the shrimp. 20000 turtles die a year in the Mediterranean as by catch. Trawlers used to encircle pods of dolphin that were chasing a school of fish and just kill the dolphin.

Steps are been taken to reduce by catch. For example trawls are now fitted with turtle escape flaps which they can open to escape, hook lines are fitted with scarerers to deter birds from coming and devise are used to submerge the lines past the diving capacity of the birds. Also eco-labelling has reduced dolphin by-catch by 90% as people want to buy ‘dolphin friendly tuna’.

Fish aren’t just over-exploited for food but 350 million fish are harvested a year from the wild for the pet trade. The target fish don’t breed well in captivity and 40% of them die before reaching their destination. However the business has a value of $963 million so fisherman are determined to get the fish and even use cyanide to stun the fish to make them easier to catch.

The over exploitation of key fish may remove prey for piscivourous animals and also remove predators that would otherwise control prey populations. This process is described as an ecosystem shift as it changes the complexity of the current ecosystem. For example in reef systems herbivorous fish like the parrot fish relay on predators to control sea urchin populations. Without this control urchin populations explode and out-compete the herbivorous fish and also lead to the bio erosion of corals. This sea urchin effect also leads to the destruction of kelp beds in California however a recently emerged market for urchin gonads has elevated the issue slightly.

The pressure of over-fishing has lowered the trophic level in many environments as the predatory fish are killed. Pauly 2005 found that biomass of large fishes is 98% lower than it is if fishing was absent. Some species like the Atlantic herring have started maturing early, they now mature at age 3 compared to age 6 in the 1950’s.

Some active fishing strategies such as trawling and dredging have extensive effects on the benthos causing mass unnatural disturbance. Trawling is where weighted nets are dragged along the seabed to collect benthic organisms. These methods use chains, teeth or water jets to dislodge target organisms and the sediment can be penetrated by 8cm. These methods destroy any habitat structure as well as collecting the organism, it has been likened to cutting down a forest to collect the deer.

Most trawling sites are naturally undisturbed so organisms are not use to this disturbance. Not only does it destroy anything on the sea bed but also re-suspends any sediments increasing turbidity which can clog up gills of fish. The benthic community changes from a suspension feeding one to a deposit feeding one.

It is clear to see that the fishing industry is far from being sustainable. The business has been treated more like a mining operation, demolishing fish stocks then moving onto the next one. Perhaps the most worrying issue for fisheries is that of by-catch. It seems barbaric to be throwing away such large amounts of dead fish back into the ocean.

Better legislation and regulation needs to be put into place to monitor shipping fleets activities to make sure they abide by a strict set of rules. Governments are increasing the amount of marine protected areas in our oceans, however that could be too little too late for many species who’s populations have been decimated. Individuals can make a difference by only choosing to eat fish which has been caught sustainably, information on this can often be found on the fish packaging, and online people can see what fish should be avoided at different times in the year in order to relive their stocks.

You Might Also Like:
bottom of page