Big Fish Little Fish – What Has Happened to Our Fisheries? 

‘Capture fishing’ is simply the removal of marine life from its natural habitat for human benefit (Swain, et al. 2007). Man did this sustainably for thousands of years, but along with an increase in global population levels, has come the habit of removing the largest fish from an area before moving on to the smaller specimens when the supply of larger fish has been exhausted (Pauly, et al. 1998). This process is known as ‘fishing down the food web’ and was first described by Daniel Pauly in his since famous paper published in 1998. Pauly’s paper went on to describe the likelihood of this causing major changes in the structure of marine food webs.

He was right. ‘Fishing down the food web’ is a problem, because over the long term the more of the large specimens we catch, the more we reduce the population of sexually mature fish able to reproduce, and that means we eventually seriously damage population levels (Barot, et al. 2002). We are, in fact, ‘shooting ourselves in the foot’.

It was the late Victorians, feeding a rapidly growing population that used up most of the stocks of large fish and left us with smaller specimens, but over the last 45 years, landings from global fisheries have shifted from large piscivorous (feeds on fish) fish, toward smaller invertebrates and planktivorous (feeds on plankton) fish, a scenario called ‘shifting baselines’ (Pauly, et al. 1998).

As a result of this long-term exploitation of wild fish stocks, negative evolutionary responses have occurred;; a reduced growth rate, age at maturation, body size and productivity (Swain, et al. 2007). A good example of this can be seen with cod. Over the years not only has the number of cod decreased but also the average body size of those caught has diminished (Barot, et al. 2002). Research has shown that the decrease in body size cannot just be attributed to the differences in age, because now even the older cod grow to a smaller size than they did previously (Barot, et al. 2007).

Subsequent to this decrease in size and abundance, fishing was ceased in several locations to allow cod stocks to recover. It was hypothesised that the fish would grow as big as they did before, but this was not the case. A study by Barot, et al. (2002) on Atlantic cod stocks along the coastline of America and Canada showed that both body size and age of cod at maturation had changed as a consequence of the selection pressure caused by fisheries (i.e. pressure to feed a hungry nation the most popular and cheapest fish available). Cod now reproduce at a younger age and at smaller body sizes (Hutchings & Rangeley, 2011).

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Figure. 1. Shows a young man in the early 20th century carrying his own body weight in cod.

We can now say with confidence that fish populations have changed dramatically over time with regards to distribution and abundance (Pauly, et al. 1998). It is essential we understand historical patterns of resource exploitation and identify what has actually been lost in the habitat, in order to develop and implement important recovery plans for depleted marine fisheries and ensure a sustainable fisheries system for generations to come.

© Eve Sanders