Key Concepts

Sustainable Fishing 

Sustainable seafood is fish or shellfish obtained from sources that use methods which consider the long-term ability of the harvested species in maintaining its population as well as the ocean’s welfare and the integrity of the whole ecosystem. Day by day the global demand for fish is growing exponentially. In response, the commercial fishing industry has increased their fishing efforts and in turn, this has led to the rapid decline various fish species ultimately leading to the collapse of global fish.


Many scientists believe that the biggest single threat to marine ecosystems today is overfishing. Overfishing can easily be defined as ‘fishing a certain species to unsustainable levels, which prevents the populations from maintaining healthy levels’. The high demand for fish is reaching dangerous levels and is having devastating impacts on marine ecosystems. Overfishing causes significant effects to the ocean and its ecosystems upsetting the delicate balance. Fish stocks all around the world are collapsing, while fish demand remains at an all-time high.
The modern fishing industry is focused on receiving a greater yield of fish, with little regard for the long-term effects. As fishing techniques become more advanced, fishing is becoming more efficient and the fish populations stand less of a chance of survival.
Over-exploitation of fisheries leads to fishery collapse, which not only harms the ocean systems but leads to loss of jobs. This is demonstrated for example when the cod fishery off Newfoundland, Canada collapsed in 1992
Top predator overfishing is a large problem in ocean ecosystems due to their high demand (eg. shark, tuna). Bascompte et al. (2006) studied population dynamics in areas of overfishing and found that unstable food webs experience trophic cascade much more than a stable ecosystem, and removal of top predators through overfishing causes this instability.
Measures should be taken based on studies of this manner to prevent overfishing causing the collapse of food web systems.

Problems with Small Populations

            Due to unsustainable fishing methods, global fisheries around the world are starting to collapse. Fish stocks are starting to reach dangerously low populations which makes these populations more susceptible to extinction due to:
1) reduced genetic variation
2) compromised social structure
3) population stochasticity
4) environmental stochasticity
This is an important issue because when a population becomes too low, it will not be able to return to its original population size with ease. Small populations may also experience what is called an ‘allee effect’ where the mean individual fitness decreases due to the low population of the species.

Trophic Cascade

The target species of the commercial fishing industry are those atop the food chain. As stated earlier, due to high demand, piscivorous fisheries such as the dolphins found in the Black Sea are experiencing heavy fishing efforts and are starting to collapse. The removal of the top predator has a major negative impact on an ecosystem’s food web (Daskalov et al. 2002). Without the top predator to control the planktivorous fish, their population will increase significantly. As a result, zooplankton populations will decrease due to increased predation which ultimately leads to the substantial increase in phytoplankton biomass.

In this case, the increase in phytoplankton biomass will coupled with nutrient addition, will likely result in eutrophication which will disrupt the food web dynamic as well as causing a decline in water quality. It can also lead to anoxia which will lead to fish kills.

Habitat Loss/Degradation

            Trawling is a method of fishing where a fishing net is pulled through the water behind one or more boats. Bottom trawling is very damaging to the ocean floor especially because it involves heavy fishing gear which causes significant destruction to coral reefs. It has been compared to clear-cutting forests since it causes a substantial decline in diversity.

Invasive Species

            Invasive species are organisms that invade a new habitat and subsequently cause an adverse effect to its ecology. It has been found that the survival of invasive species is negatively correlated with the species richness of a habitat (Stachowicz et al, 2002). However, unsustainable fishing methods cause a significant decline in species richness and diversity. This decline in diversity makes fish populations more susceptible to invasive species. Competition for resources will become more intense and it will most likely lead to a further drop in species richness.


Some fishing methods are damaging to the ocean because they have high levels of bycatch. Billions of marine creatures are caught by fishing boats, and are discarded back into the oceans. These “non target species” do not have specific commercial value in these cases; therefore they are killed for no purpose. Bycatch is one of the largest threats to marine ecosystems, and can be prevented by only supporting fish caught from organizations that take measures to prevent bycatch. 
In studies done by Read et al. (2006), it was found that bycatch levels of Marine Mammals in the United States is high (aprox. 7000/yr between 1990-1999). Upon implementation of reduction efforts, a drastic decrease in bycatch levels was observed. It is important to support fisheries that follow bycatch reduction regulations in order to help prevent unnecessary bycatch of other fish or mammals.

Aquaculture is the process of farming fish or plant populations in fresh or salt water, generally for consumptive purposes. This is often done in open net cages, which are placed directly in the ocean. This method can cause a series of problems for wild stocks from waste production, parasite transfer, entanglement, escapees, habitat destruction, use of antibiotics, etc. A suitable alternative to this practice is using enclosed tanks, which minimizes effects to local populations.
The right seafood choice may be affected by what method a certain fish was farmed in, if it is a farmed stock. 
Mercury is a naturally occurring substance. Plants absorb mercury from soil and air during growth, bacteria can convert inorganic mercury into organic compounds, and animals can ingest mercury. All animals contain trace levels of mercury. Mercury is known as one of the most dangerous environmental poisons. Exposure to mercury also causes a number of negative effects in the human body from the nervous system, digestive system, skin, eyes, respiratory, endocrine system, and cardiovascular system (Viranen et al. 2005).
Health Canada warns that you should try to reduce your exposure to all forms of mercury whenever possible. People who consume large amounts of fish, marine mammals and wild game as part of their daily diet increase their risk due to bioaccumulation up the food chain (Matthews, 1983). Pregnant women should be especially careful as exposure to mercury can have greater effects on a developing fetus.