The causes and effects of dropping fish populations

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The causes and effects of dropping fish populations


The aim of this investigation is to investigate and discuss the causes and effects of dropping fish populations.

The Underlying biology:

There are a number of factors to consider when discussing the drop in a population count of any species. One of the main factors is overexploitation. To ‘exploit’ a resource is to take advantage of and consume it at a rate that is not harmful to the overall population that is being exploited, sheep for example, we take wool off their back and kill them for food thereby exploiting them, and yet they are not in danger of becoming extinct because we are careful about how many we kill. Fish on the other hand were, not so long ago, fished without fishermen being regulated in how many fish they could catch. In more recent years (the past decade or so) it has been realized that the fish are not reproducing at the same rate that fishermen are exploiting them therefore their population has dropped significantly in the past few decades. This is a prime example of overexploitation.

Another factor to consider in the dropping number of a species population is the damage that is being done to its habitat. There are 3 forms that habitat loss can come in: habitat destruction, fragmentation and degradation.

  • Habitat destruction is when a habitat, such as an area of woodland, is decimated. The destruction can be caused by a number of things. Natural disasters can very quickly destroy a habitat, however in more recent years humans have been the cause of habitat destruction to make way for developments such as roads and buildings.
    • Habitat destruction can happen in fish species during long periods of drought as this will dry up a body of water that they are living in, also natural disasters such as hurricanes have been known to decimate fish habitats such as ponds and lakes.
  • Habitat fragmentation is when a habitat is separated into several smaller parts and members of the same species are separated by barriers that they cannot cross. Fragmentation can be caused by humans or by nature, forest clearing and newly formed rivers respectively for example.
    • While it is difficult for a fish’s habitat to be fragmented it is theoretically possible for a group of fish to be split up by currents which would have the same effect.
  • The last form of habitat loss is degradation, this is where the quality of an environment is reduced to a point where it becomes impossible for a species to live there leaving them with either the choice of attempting to become a naturalized species, moving to a new habitat and hoping that it can sustain them, or remaining in their current environment and most likely dying out.
    • Habitat degradation happens in fish species when their habitat is disrupted by outside factors such as chemical dumping. This example would, if not killing the fish instantly, disrupt the aquatic food chain by killing the algae that smaller fish feed on, having a knock on effect to the higher-in-the-food-chain species.

There are two types of environmental damage: direct and indirect. Chemical dumping is a form of direct environmental damage. An example of indirect damage is the greenhouse effect; this is where ‘greenhouse gasses’ such as methane and carbon dioxide that come from burning fossil fuels and industrial processes get trapped under the Ozone layer and they in-turn trap heat under the Ozone layer, this has a number of adverse effects on the worlds natural environments.

  • In fish, species that are sensitive to temperature changes are forced to leave their usual habitat in the hope of becoming a naturalized species somewhere where the water is at an acceptable temperature. The problem with this is that there is no guarantee that a large enough food source would be available to them. Even in the situation where there is some food available to them; if it is not equal to or greater than their original source the population will drop, this will have a negative long term effect as it will decrease their genetic diversity through the ‘bottleneck effect’.

To understand the bottleneck effect we must first understand what is meant by the term ‘genetic diversity’. Genetic diversity is the genetic variation across a whole species. Take humans for example, we can vary in many ways; height, eye colour, skin colour, hair colour even our foot size, it’s fair to say that we are quite a genetically diverse species. Genetic variation happens as a result of mutation (random changes in the structure of a gene).

  • Fish on the other hand, while having a large population like humans do, do not have much visible genetic diversity, this can be explained by the ‘bottleneck effect’.

For the bottleneck effect to take place there must be a sudden significant drop in the population. Taking out a portion of a species takes out all of the unique traits that those in the species that were killed were carrying in their DNA. The bottleneck effect has 2 main adverse effects. Firstly, due to the loss of genetic potential any severe change in their habitat can’t be adapted to as none of the members of the species have genes capable of doing so. Secondly, the bottleneck effect leaves the population with members who are so genetically similar that breeding becomes very much like inbreeding, which is dangerous as it leads to recessive alleles.

  • The bottleneck effect has certainly had an effect on fish populations due to large amounts of overexploitation over the past few decades; however there is no way of measuring exactly how much genetic potential has been lost.

Selection of data:

I have taken data from 2 sources

Source 1: An I.C.E.S. (International Council for Exploration of the Sea) report on Cod biomasses in the North Sea, Eastern English Channel and Skagerrak.

We know this source is reliable because I.C.E.S. is an organization ran by scientists purely for scientific discovery and recording, they have no reason to manipulate this data.

Source 2: A Scottish Government report on demersal[1] landings into the UK by UK and foreign vessels from 1938 to 2013 (table 3.7)

We know this source is reliable because the smaller parts of data that make up this report have to be relied upon in court when a fisherman is accused of fishing above his quota, this ensures their accuracy.

Analysis of data:

If we firstly look at the line graph of the data taken from source 1. From the years 1990 - 2000 we see a clear trend of the biomass yo-yoing up and down year after year almost perfectly. This is most likely because each year fish stocks would be evaluated and after seeing that they had significantly dropped from the previous year (due to overfishing) they would heavily restrict how many fish could be legally caught, therefore giving the fish time to repopulate the ocean. Then the fishermen would overfish the next year after the quotas had been slackened and that cycle would repeat until the government began to think of long term sustainability of fish populations. Looking at the same graph from the years 2000 – 2011 we can see that there is a very steady rise and fall in fish biomass, nowhere near as extreme as the 1990 – 2000 range. This means that fishing quotas were being created with the preserving sustainable fish stocks for years to come. And finally, if we look at the 2011 – 2014 range we can see that fish stocks are beginning to climb, this is most likely because cod quotas had been tightened further to make sure that the fish are reproducing faster than they’re being caught, allowing the stock to permanently be replenished.

Secondly, look at the line graph of the data taken from source 2. From the years 1938 – 2000 we can see that there is a steady decline in the quantity of fish landed in the UK, this is due to the fishermen’s overexploitation (the fish weren’t breeding as fast as they were being caught) was happening for decades. Until (as mentioned in the previous paragraph) the government stepped in in 2000. If we look at the line graph form the year 2000 – 2013, we can see that there is nearly no change year by year, this is because the fishing quotas ensured that the amount of fish caught would remain consistent year by year.


The aim of this investigation was to “investigate and discuss the causes and effects of dropping fish populations”.

We have discussed the effects, namely the bottleneck effect. The causes, namely overexploitation, habitat destruction, habitat fragmentation, habitat degradation and climate change.

We have also investigated the causes and effects through examination of our 2 line graphs. From these we have concluded that over fishing and a lack of forward thinking on behalf of the government in recent decades have been the main causes of serious drops in fish population, and as a result, a lack of genetic diversity courtesy of the bottleneck effect.

[1] Demersal fish types include the likes of Cod, Haddock and Halibut