In our last issue we discussed different factors which influence resource Maximum Sustainable Yield (MSY). We tried to explain why the MSY of any living renewable resource is not a constant but rather can be increased or decreased depending on the mix of sizes of fish being targeted. We explained that the relationship between the MSY and the average size of fish landed is not linear (just as the relationship between the resource sustainable yield and the size of the resource is not linear).

In this section we would like to expand this discussion to include the economics of fishing. Bioeconomic is the discipline that combines concepts about the biological productivity of a resource and the economics of commercial fishing. The bioeconomics of living renewable resources is the essence of fisheries science, distinct from simply the biology or the population dynamics of marine resources.

Fisheries science involves the rational and quantitative trade-off of biological risk against economic return with the overall aim of ensuring profitability for the fishery without compromising the long term productivity of the resource. An important result that flows from bioeconomics is that managing stocks so that they produce MSY is not necessarily the most sensible approach from an economic point of view.

Variation in productivity between different resources

The reason that different fisheries have different maximum sustainable yield levels is a combination of the level of recruitment and the yield-per-recruit. The maximum recruitment that can occur is limited by the carrying capacity of the environment. Yield-per-recruit is determined by the effects of natural mortality, body growth rate and the size selective characteristics of the fishery.

So for example, recruitment in the South African anchovy fishery is very high, but so is natural mortality. However, the fishery is a recruitment fishery (selects small fish), so that it is not so heavily affected by natural mortality, since fish are removed before too many die naturally. Alternatively, somatic growth rates in hakes are high, but natural mortality is low, so a fairly productive fishery can exist, even though fishing only takes full effect on ages of three years or more.

Although a species may be successful and abundant in its environment, this does not necessarily mean that a productive fishery for it can develop. For example, the South African west coast rock lobster was super-abundant in its pristine state, but is not thought capable of producing more than at most 5 000 tons a year under optimal conditions, since it is a very slow growing species.

Marine resources are inherently very variable. This variability affects catches, and is due to a combination of factors. One of these is the high degree of variability in the size of the recruitment and natural mortality streams, which causes fluctuations in the resource biomass, and hence catches. Other factors which affect catches are variability in the behaviour of fish, which might move from one place to another, with the result that their availability to the fishing gear at a particular place and time may vary. Even if they are available at the right time, they may not respond to bait or other gear in the expected way.

Catches will obviously also be affected by the amount of effort expended in the fishing operation. This is perhaps the only factor that can be controlled by fishermen. All the others are a feature of the resource, which make management much more complicated that it might seem at first.

Cost of fishing: fishing effort versus resource size

There are a number of terms that are used in the bioeconomics of marine resources. Some of these terms are new and need to be defined.

Fishing effort is the quantity of gear used to fish, multiplied by the time that the gear is in use. Typical units of effort are: angler hours, trap days, net throw hours, trawl hours or purse seiner days. An effort of 30 boat days can be achieved by a large number of combinations of number of boats used, and number of days fished, e.g. 3 boats and 10 days, 10 boats for 3 days, 5 boats for 6 days, 2 boats for 15 days, etc. Each of these combinations represents the same amount of fishing effort.

We consider a simple analogy to illustrate the interrelationship between fishing effort, resource biomass, catch and harvesting costs. The fishing process can be compared to the process of using a sieve to remove particles of from a container of water. The following relationships must apply:

• i) catch (number of particles removed) is proportional to fishing effort (the time spent to remove these particles) for a given resource biomass (number of particles in the container),
• ii) catch is proportional to resource biomass for a given fishing effort,
• iii) from (i) and (ii) it follows that resource biomass is proportional to catch divided by effort (number of particles removed per unit of time),
• iv) fishing effort is proportional to the ratio of catch to resource biomass, i.e. fishing effort is proportional to harvest proportion (the number of particles removed compared to their initial number in the pool),
• v) harvesting costs (wages per hour of work) are proportional to fishing effort

The relationship between effort and catch and the appreciation that the running cost of fishing is proportional to fishing effort, tells us that for the same catch, costs are lower at higher levels of resource biomass. Does this means that one would like resource biomass to be as large as possible? The answer is of course no, since one also needs to consider whether by increasing resource biomass, the reduction in sustainable yield (long term catch possible at a stable biomass) is more than compensated for by a reduction in harvesting costs.

Given this complication, what is an appropriate target biomass which provides a sensible economic management framework which is compatible with the biological preservation of the stock? From the above discussion it is clear that one should aim at a point at which the combination of resource productivity (sustainable yield) and cost of harvesting leads not to Maximum Sustainable Yield but rather to Maximum Economic Rent.

This is at a biomass level which is somewhat larger than the biomass which generates MSY. The position of maximum economic rent on the curve describing the relationship between sustainable yield, cost and resource biomass, and how to manage a resource to reach this point, is dealt with in our future articles.