The Marine Trophic Index (MTI) is an indicator of the status of a marine ecosystem. It is a measure of the average trophic level (TL) of fish taken in a given year.
A trophic level is a level in a food chain that an organism occupies. The trophic levels indicate who eats whom in a food chain, with large fish such as cod occupying a high-trophic level and organisms such as plankton occupying the bottom-trophic level.
Higher-trophic level species like cod and tuna are typically the more sought-after fish. A catch that contains mostly lower-trophic level fish suggests that higher-trophic level fish populations are depleted.
To determine the MTI, one identifies the trophic level of all fish taken (how many at trophic level 3, trophic level 4, and so on) and then calculates the average trophic level represented by all fish taken that year.
According to the catch data from the Newfoundland-Labrador Shelf, catches from 1950 to around 1990 mostly contained high-trophic level fish. The average trophic level then declined quickly. From the mid 1990s onward, catches contained mostly low-trophic level fish.
Consider what happens when cod are overfished. One of the obstacles to the recovery of cod and other large predatory fish is the loss of their own prey.
When cod are depleted, fishers pursue the herring, crabs, and shrimp at lower-trophic levels. The loss of prey thus reduces the food supply for the cod, which ultimately jeopardizes the cod recovery.
Additionally, the loss of higher-trophic-level predators allows some lower-trophic-level species to increase in number.