Trends in Average Length of Commercial Landings Over Time

Commercial Landings Average Lengths Over Time

As fishers remove cod of specific sizes from the resident population in the Gulf of Maine, the average size of the remaining cod is altered. Depending on whether the larger or smaller cod are constantly being removed the average size of the remaining cod will change. This can be shown in the graph below of the change in average length by age of commercially landed cod in the Gulf of Maine since 1982.

Graph created by E. Brazer Jr.

From 1982-2000 data was only available for age-1 cod in 1982, 1984 and 1987. During these three years the average length of age-1 cod decreased from 43 cm (1982) to 39 cm (1984) and then increased again to 47 cm (1987). During the same time frame the average length of age-2, age-3 and age-4 cod increased from 48 cm to 54 cm, from 54 cm to 61 cm, and from 63 cm to 66 cm, respectively. Age-5 cod decreased in average size during these 18 years from 77 cm to 75 cm. Age-6 cod also decreased in average size from 86 cm to 82 cm. The same trend of decreasing average size occurred in age-7, age-8 and age-9 cod (95 cm to 81 cm, 98 cm to 93 cm, and 107 cm to 102 cm, respectively). For the age-10 year class of cod, data was unavailable for the years 1993, 1996, 1999 and 2000. However, from the data that was obtained there occurred an increase in average size from 101 cm to 126 cm. Age-11+ cod also saw an increase in average size from 121 cm to 127 cm, but data was not available for this year class for 1992, 1993, 1995, 1996, and 1998-2000. Averaging this data together we see that the size of an average commercially landed cod has increased from approximately 60 cm in 1982 to approximately 68 cm in 2000.

An average increase in the size of a fish over time in indicative of fishers removing mostly smaller fish from the population. Studies have shown that constantly removing the smallest of the species every generation produces a population of fish with a larger-than-average size after a few generations.[37] This occurs because after removing the smaller individuals only the larger individuals are left to reproduce. In general, larger individuals produce larger offspring, and isolating the largest of these offspring throughout the generations creates a population of naturally "large" individuals.[38] This appears to be what is happening with the cod in the Gulf of Maine. Fishers are removing a proportionately-larger population of small-to-medium individuals (because there are more small-to-medium fish than large fish) and are inadvertently increasing the average size of the cod over time.