“Before they grow so big, the baobabs start out by being little."

From “The Little Prince” by Antoine de Saint Exupéry

Ask the man in the street what comes to mind in regard to the “adverse” effect of fishing on the environment and he will say overfishing. When you press him to define what is meant by overfishing, he will say that it involves the unsustainable exploitation of marine resources, driving them to biological extinction. However, as explained in a previous article, sustainable fishing is not necessarily synonymous with sensible fishing, and therefore it would appear that overfishing may be associated with certain undesireable forms of sustainable fishing.

So what is overfishing? Is this a single state or thing, or does it cover a number of possible resource states or outcomes? Is overfishing a strictly biological concept or an economic concept? And perhaps most importantly, how does one reverse the slide towards overfishing, or better, how does one prevent resources from being overfished in the first place? These and related issues are addressed in this article and in the article that follows.

The term overfishing is often erroneously assigned to a great number of different fisheries or to a nation’s fisheries in general, without any discrimination made about the specifics of the fishery or the particular species involved. This is partly because (a) most exploited resources experience dramatic changes in their biomass, and the size of individuals in the population, over the history of the fishery, and (b) there is little appreciation of what overfishing actually means. For example, a severe case of overfishing may exist even though there is no risk whatsoever to the persistence of the exploited species. On the other hand, sustainable and economically viable fisheries can take place under conditions of a severely depleted resource biomass.

It is worth noting that very few, if any, cases of exploitation leading to species extinction have been recorded for marine fisheries. This is because long before a population faces any real prospect of extinction, it becomes too difficult and expensive to continue fishing. It should be noted that there are a number of documented cases of extinctions of marine mammal species. This highlights the fact that marine mammals can be depleted to very low levels by commercial operations for a variety of reasons related to their body size and behaviour, e.g. to name just one factor, they have to break the surface of the water to breathe.

Table 1: An hypothetical example of the change in the number and biomass of all fish in a particular cohort as they age. The last column indicates the ages at which growth overfishing, optimal fishing and growth underfishing occurs.

Age
(year) Individual
weight (gr) Cohort
Number Cohort
biomass (kg) Fishery type
1 3 100000 300
2 63 40000 2520 Growth
3 228 18000 4104 overfishing
4 423 9000 3807
5 1263 4950 6252
6 2298 2970 6825
7 3693 1931 7129 Optimal
8 4473 1351 6045 range
9 4923 1081 5322
10 5073 973 4936
11 5133 876 4495
12 5163 788 4069 Growth
13 5178 709 3673 underfishing
14 5187 638 3311
15 5193 575 2984

Growth overfishing and "growth underfishing"

In order to simplify the explanation of “growth over-fishing” versus “growth underfishing”, refer to Table 1. This shows an example of how fish of a particular cohort grow and die as they age. Growth adds biomass to a cohort, while death removes biomass. The cohort biomass at any age is the number of fish that have survived to that age, multiplied by their body weight. As is demonstrated by the table, initially the cohort biomass is increasing with age, however as time passes, more biomass is removed by mortality than added by growth and the cohort biomass starts to drop.

Let us assume a simple case in which a fishery is controlled by a minimum legal size (e.g. typical of lobster and abalone fisheries in South Africa) and most of the fish above this size are removed by the fishery. In this situation most of the annual fishing yield comes from cohorts which are below the legal size the previous fishing season and grow to exceed the minimum legal size before the start of the current season.

Growth underfishing would occur if the size limit is set higher than the size at which the maximum cohort biomass is reached. From the table, the yield at an age of 10 years would be 30% lower than the yield at an age of 7 years.

Growth overfishing on the other hand occurs when the fishery targets a size below the optimal size. In such a situation fish are removed from the sea before the cohort has had the opportunity to achieve its maximum biomass level. For example, a fishery which catches fish at 3 years old will lose approximately 42% of the potential yield that could be achieved by catching them at 7 years old instead. Calculation of the actual optimal fishing age is more complicated than this in practice because one is never able to harvest an entire cohort, the harvest will consist of a range of cohorts and sizes.

It is also very important to realise that growth underfishing can occur even though there may be no biological risk to the resource.

Recruitment-overfishing

Before discussing recruitment-overfishing, it is necessary to distinguish between two common definitions of recruitment. The first is the recruitment of new-born juveniles to the population and the second refers to fish which move (i.e. grow) from the unavailable portion of the stock (either too small, or inaccessible for some other reason) to the stock which is available to commercial fishing gear. For the purpose of this document we use the term 'juvenile recruitment' to refer to the first, and 'commercial recruitment' to the second.

The previous discussion of growth overfishing does not address the issue of how spawning and recruitment is affected if too many large mature fish are removed from the sea. This is because growth overfishing, although normally associated with a substantial reduction in spawning biomass, may not necessarily reduce commercial recruitment.

As was indicated in the section on the relationship between spawning biomass and recruitment, most living marine resources are surprisingly resilient to severe declines in spawning biomass. It is well accepted that for many fisheries the spawning biomass can drop to as little as 20% of its pristine size without any measurable effect on commercial recruitment. Although the mechanism which causes this is not known, it seems likely that it is the net outcome of a myriad of different processes, including, for example, complex changes in the fecundity of mature females and the mortality of eggs and larvae.

Having said this, if spawning biomass does indeed fall below the 20% threshold mentioned, then the experience is that commercial recruitment will be reduced. Below this critical spawning biomass level, recruitment overfishing has occurred.

In a future article the difference between biological and economic–overfishing and the economic implications of overfishing will be discussed, including the economic cost associated with running a risk averse fishing operation.

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