| Discussion paper regarding the biological basis for regulating rod numbers to manage fisheries effectively1. Fishing effort: how many rods should be permitted? 1. Basic principles · Regulation is required to prevent overfishing. · Effort may be regulated by controlling numbers of anglers, their efficiency and the length of the fishing season. and the length of the fishing season · Efficiency may be regulated by controlling methods, such as types of bait and approach (eg legering vs spinning), and number of rods. · There may be rules to say what, if any, fish may be kept, and what fish must be released. rods 2. Basic distinctions between life history characteristics of the foray species The primary distinction among species, with regard to regulating fishing effort, is probably not whether they are "game" or "coarse" fish, but whether cohorts of the species tend to be vulnerable to the fishery over successive or single years. In this respect, salmon tend to be vulnerable over a single year when they return from the sea to spawn, whereas trout, cyprinids and pike are vulnerable over multiple years. Of course, some salmon spawn several times as anadromous fish and may be taken by the fishery on multiple occasions, but this is a relatively rare occurrence and in the present context the species can be classified as single spawners. Spawning as salmon parr is irrelevant to the arguments. In the case of multiple spawning species, large, old fish tend to be particularly valued and may often be the main determinant of the quality of a fishery. 3. Single spawners Principles of fisheries regulation are well established for single spawners. There tends to be a bottleneck in the freshwater stage of the life cycle such that there is often an excess in numbers of adults above that required to spawn and saturate rearing capacity. This excess can support a sustainable fishery. In theory, it is possible to manage populations efficiently by cropping below the minimum number of adults needed to saturate rearing capacity, but there is also a strong argument for incorporating large safety margins for uncertainties in stock size due to inter-annual variations. Generally, the catch of salmon by rod-and-line is estimated to be of 5-20% of the stock. There has seemed to be little need to regulate numbers of rods permitted per angler because multiple rods with set baits are generally not efficient methods for catching salmon, with the exception of harling. It appears that, historically, exploitation has generally been easily accommodated leaving an abundance of spawners. In principle, an increase in angling efficiency would be of concern only when stocks are particularly weak, as is the case at present. 4. Multiple spawners In the case of multiple spawners, each cohort is vulnerable to the fishery for several years before the fish are of a size that is often of real value. During each year of growth, a fraction of the cohort is killed due to angling and the catchibility of a further fraction may be reduced due to captured (and subsequently released) fish learning to avoid baits in the future. In the context of the present discussion, fish that have learned to avoid baits can be deemed to be dead to the fishery. Even with a "no-killing" policy, some angled fish die. Some fish suffer inadvertent lethal damage from hooking. Other fish die due to the physiological exertions of being captured. This process, which can be delayed, is well-known to biologists and can be accounted for by a lethal build up of poisonous lactate in the muscles. Other fish are likely to succumb to predation when in a weakened condition after having been caught and released but separated from the shoals that normally provide them with protection. A simple steady-state model can be used to demonstrate the way that the interaction among fishing effort, fishing mortality expressed as the percentage of captured fish that die (F) and other annual natural mortality (M) can influence the catch of fish of different age classes in multiple-spawning fish. It is assumed (1) that eggs are spawned in excess of a carrying capacity determined by a bottleneck early in life; (2) that there are no interactions among year classes; (3) F and M are constant throughout life. Variation in fishing effort can be through regulating either or both numbers of rod/angler and numbers of anglers. Note that doubling the number of rods is not necessarily the same as doubling the number of anglers. M is set at 0.1 (that is 10%) per annum in the example shown. In each of the graphs attached, the series of lines shows values for cohorts within the population with the youngest recruited to the fishery at the top and the oldest at the bottom. Type I response When F is relatively low, as shown for a value of 0.01 (Fig. 1a), it has little influence on year class strength, that is, the lines are nearly flat. Catch per unit effort decreases with age of the fish because predominantly M reduces the size of the cohort from one year to the next. Under these circumstances, it makes good sense to increase fishing effort because there is then an increase in the total catch of fish from all cohorts (Fig. 1c). This type of response (Type I) is probably typical of many carp fisheries. Type II response When F is increased, in the example shown to 0.04 (Fig. 2), there is a marked decrease in year class strength with increase in fishing effort and this effect increases with age (Fig. 2a). Catch per unit effort shows similar trends to those for cohort strengths. The total catch of young fish increases with fishing effort. However, the catch of older large fish increases to a maximum and then declines as fishing effort increases. Type III response When F is increased further, for example to 0.1 (Fig. 3), increase in fishing effort results in an increase in the total catch of small fish. However, any increase in effort inevitably reduces the total catch of large fish. The response of the relationship between total catch and fishing effort is curvilinear. Therefore, a unit increase in effort from 2-3, has a greater reduction in catch than an increase from 1 to 2. 5. Managing fishing effort on multiple-spawning fish The requirement for regulation of fishing effort depends on the values of F and M. For Type I fisheries, there are significant gains to be made from permitting multiple rods because catch per rod and total catch increase. In the case of Type II fisheries, a balance must be struck between fishing effort and catch of older year-classes. For example, for the model depicted in Fig. 2, if 100 anglers constitute 1 unit of fishing effort, they could increase their catch of old large fish by using sufficient rods to double their efficiency. However, any further increase in the numbers of rods per angler would actually reduce their total catch of old fish due to the increased mortality on small fish. For Type III fisheries, any increase in angling effort has a cost in terms of the numbers of large fish captured. 6. National versus local management In the case of Type I fisheries there is clearly no need to regulate fishing efficiency of individual anglers except to minimise the chances of increasing F, for example, through deep-hooking fish that take baits simultaneously on multiple rods. Arguably, this may best be achieved at a local level taking into account specific fishery conditions and experience. Alternatively, or additionally, a national maximum number of rods could be set and fisheries could impose further restrictions locally. In the case of Type III and some Type II fisheries, there is a dilemma that permitting the use of multiple rods will reduce the total catch of large fish. Note that this is not just fish per rod, but fish per angler. Unfortunately, this effect is counter-intuitive: it can seem on the face of it that everyone can increase their catch rate by increasing the numbers of rods that they use, but this is not the case. Of course, on a fishery where nearly everyone uses a single rod, the individual who uses two rods can increase his/her catch rate without the major reductions in densities of older cohorts that result when many anglers use two rods. There is a case for national regulation of angling effort through numbers of rods permitted in order to maintain the quality of Type II and III fisheries. The problem with local management is that fisheries might be forced to allow multiple rods in order to compete effectively in a market driven by individual desire to maximise catch and an intuitive perception that multiple rods must be better than one. If this case is worthy of taking further, then issues of classifying fisheries into Types need to be considered. John Armstrong FRSFL 1 September 2004 |