INTRODUCTION
Reduction of wild stocks of Atlantic salmon, Salmo salar, through heavy exploitation or through loss of habitat, has encouraged managers to try to compensate for these decreases by stock enhancement. When catches of salmon in a river are less than expected, or of a quality assumed that the river is underpopulated (or will be), and that the simple remedy must be to increase future numbers by stocking that river with more juveniles. Is this assumption valid?
To obtain a representative index of changes in numbers, sizes and ages in a salmon population, it needs to be fished at the same intensity on all its component parts over a period of years. Close times and close seasons restrict fishing to an arbitrary, and thus unrepresentative, fragment of the population. Even this fragment varies, since whether fish are there to be caught at a particular time depends on both ocean and river conditions, which vary from year to year. So, by themselves, numbers in the catch are not normally a reliable indicator of the numbers in the stock.... Likewise, the age and size composition of the catch are characteristics which do not necessarily reflect accurately those same features of the stock.
Hence assumptions about stock based on the partial samples normally provided by catches are not sound . How, then, can one assess the status of a particular stock?
The managerâs objective is to ensure that there is a continuing adequate supply of harvestable adult fish returning from the sea. He will achieve this only if sufficient juvenile fish enter the sea from the nursery grounds. Future catches, as with all fisheries, depend on successful production of juveniles. Hens predictions about the quantity and quality of future catches depend on knowledge of the quantity and quality of the juveniles which will yield the adults that make up those catches.
STOCK DISCRETENESS
Natural selection ensures the success of a species by favouring reproduction among animals which are adapted harmoniously with their environment, and by eliminating those not so well adapted. Every river system is different. It imposes physical and biological constraints on the animals developing within it, which differ in subtle ways from those imposed in other rivers. Hence the pressures of natural selection differ between rivers, and it is therefore not surprising to find that the genetic composition of the salmon population in one river differs from that in the next. This close match between genotype and environment is preserved efficiently through high homing precision, so that these stocks remain discrete. So the stock of salmon of a particular river is likely to possess a genetically determined range of patterns of development, tailored by natural selection, which ensure success in that specific river. If salmon are to be stocked into a system to increase the numbers of an existing population, then ideally they should be of the same genotype as the recipient population. If they differ, subsequent interbreeding could modify the genetic spectrum or that recipient population. Taggart and Ferguson (1986) have documented just such changes in the brown trout S. trutta population of L. Erne, following the stocking of over 8 million Movanagher trout over the period 1968 -1983. It has been shown that species such as salmonids whose populations are subdivided into discrete stocks are particularly vulnerable to directional changes in genetic composition (Thorpe and Koonce, 1981). Such changes could impair the ability of the population to respond to environmental change. That this is more than a theoretical possibility has been illustrated by the records of a stocking exercise with chum salmon, Oncorhynchus keta, in Sakhalin. Altukhov (1981) reported that 350,000,000 fertilized eggs were transferred from the Kalininka River to the Naiba River between 1964 and 1971. Before the transfer, the Naiba river carried a spawning population of about 650,000 chum salmon. By 1969-70, the genetic characteristics of the stock returning to the Naiba has shifted towards those with the Kalininka fish, and by 1980 the returning population had decreased to 30,000-40,000 spawners. By 1985 the population was virtually extinct (Yu. Altukhov, personal communication, 1986). He concluded that this local disaster was the result of the massive genetic migration of non-adapted genotypes.
Similarly but less dramatically, Smoker (1987) recorded that the return of native chum salmon to a site in Alaska was twice as great as transplanted stock, but from hybrids between the two the return was very small indeed.
Apart from differences in biochemical genetic markers (of unknown functional significance), the differences in biology between all these various chum stocks are unknown. It is not know, therefore, at what stage of life history the hybrids failed through lack of appropriate adaptations.
These examples are all the more important for Atlantic salmon managers, since the genetic variation between stocks of Salmo species is know to be very much greater than that between stocks of Oncorhynchus species (Ryman, 1983; Altukhov and Almenkova, 1987). (Note: this study was publish prior to merging steelhead with salmon under Oncorhynchus. Steelhead at the time of this study were considered to be a Salmo). If transfers of the relatively similar chum salmon stocks can interfere with one another so adversely, how much more damaging could be transfers of Atlantic salmon stocks? When ranched Atlantic salmon in New Brusnwick were hybridized between stocks, Bailey (1987) recorded a decrease in return rate in the second generation. He suggested that these inter-stock pairings may have interfered with the genetic components of navigation, and so impaired the fishes homing ability.
Atlantic salmon have been stocked repeatedly in many river systems containing supposed depleted populations, throughout their natural range for a very long time, and the fish are still there. This is encouraging, but current scarcity of evidence for adverse effects (e.g. Moller, 1970, but also Bailey, 1987) similar to those seen in the Naiba River chums is not evidence of total absence of such effects in Atlantic salmon enhancement. The real long-term success of augmentation in this species has not been measured, and would require a study similar to that made in Kamchatka over a period of 20 years. However predictive pointers do exist:
1) Stocking have often been total failures;
2) Natural strays make negligible impact on recipient stocks; and
3) Multiple distinct stocks do coexist in some systems.
To take these points of evidence in turn, firstly, failures: Saunders (1981) pointed out that many salmon stocks have been eliminated due to human competition for the use of their native rivers. Restoration of these vanished populations has been exceedingly difficult to attain, for example in New England, and he suggested that important genetically based behavioural traits had been lost, especially in relation to appropriate run timing of smolting and spawning fish. Baileyâs (1987) data would add navigational control to this list. Similarly, Altukhov (1981) reported many instances of complete failure of chum salmon transplants. Taggart, Ferguson and Mason (1981) suggested that since similar genotypes often existed in contiguous geographic areas, failure of stockings of Îgeneralized or mixedâ strains of hatchery trout could have been due to lack of local genetic adaptation.
Secondly, straying: Rasmuson (1968) predicted that a straying rate of only 2 per cent per annum would be enough to prevent genetic differentiation between Atlantic salmon populations. The few estimates of straying rate in Atlantic salmon suggest that it is greater than this, at about 2-7 per cent per annum (Thorpe and Mitchell, 1981; Browne et al., 1983), but Hansen and Lea (1982) have reported up to 17.6 per cent in some Norwegian rivers. However, Stahl (1981) demonstrated clear genetic differentiation between stocks from biochemical evidence in a group of six Swedish rivers. In these rivers, 2 per cent straying represented 50-200 fishes; but Stahl (1981) calculated from his examples that in fact less than one stray salmon per year had contributed to the progeny of subsequent generations. Hence differences of developmental timing between stocks, or behavioural inhibition, may have diminished the chances of natural hybridization in these river stocks.
Thirdly, multiple stocks: Riddell, Leggett and Saunders (1981) noted the separate nature of the Atlantic salmon populations of different tributaries of the Miramichi. Stahl (1981) found distinct biochemical differences between fish of the River Lainio, and the River Torne, to which it is a tributary. Altukhov (1981) reported the existence of 30 different spawning stocks of sockeye salmon, Onchorhynchus nerka, in Lake Azabachye, Kamchatka, defined by biochemical genetic differences. Ryman and Stahl (1981) reported many examples of stock differentiation in Arctic charr, Salvelinus alpinus, within Scandinavian lakes, and in brown trout in lakes and even within small streams. Ferguson (1980) and Ferguson and Mason (1981) found evidence for coexistence of three reproductively isolated and genetically distinct stocks of brown trout in Lough Melvin. Crozier and Ferguson (1986) have found two distinct trout stocks in Lough Neagh, each of which is made up of components associated with several spawning streams, such that there is examples where given to the STOCS Symposium (1981).
To summarize these findings, much foreign material would be useless for stocking into a river since it would fail to return; some might be of value only once, if after return it failed to breed at all; and some could be useful, as long as its breeding behaviour and occupancy of the river environment complemented, but did not interfere with, those of existing stocks, so that there was no reduction or dilution of essential genetic information within the population.
NATURE OF STOCK DIFFERENCES
The developmental programme for an organism is genetically determined, but it runs under environmental instruction. Major changes in development, such as smolting, and emigration from freshwater, are influenced environmentally by changes in daylength and temperature. It is likely that these indicators of season (especially daylength) act as synchronizers of physiological change (Saunders and Henderson, 1970; Lundqvist, 1983), and so of the changed behaviour patterns which ensue (Wagner, 1974). Natural selection will have determined the optimal time for such changes for any given stock: those fish that make their changes at the optimum time will survive to grow and reproduce; those which change at other times will be eliminated. Timing of events such as emigration from rivers must relate to optimal protection from predators, optimal physical conditions for downstream transport, and optimal physical and biological conditions for entering the sea. In large southern river systems like the Connecticut River this would include arrival in the ocean before sea temperatures reach lethally high levels. Such a timetable will be specific not only to individual river systems, but to separate regions of those systems.
While rearing experiments have shown that developmental rates are heritable characteristics in salmon (Thorpe 1975; Thorpe and Morgan, 1978; Bailey, Saunders and Buzeta, 1980; Thorpe, Morgan, Talbot and Miles, 1983), the expression of those different genetic capacities is heavily dependent on local environmental conditions. Hence, the life-history strategy adopted by successful stocked (hatchery) fish is likely to resemble that of the native stock.
But beside the inherent genetic differences between stocks, the expression of those differences depends upon environmental opportunity.
...the first measure should be to augment with the existing native stock. Consequently, the ideal material for stocking a system is that which is native to it. Since each salmon stock consists of diverse components, augmentation should aim to increase all those components in proportion to their occurrence in the initial population. To reduce the possibility of impairment of genetic diversity by inbreeding, at least 30 males and 30 females should be used as the founding broodstock of each stock component (spring, summer, fall migrants for example).
Salmonid fish species are composed of genetically distinct, reproductively isolated stocks, finely adapted to the specific features of the environments in which they develop. Interference with this close match of genotype and environment through introduction of foreign stocks is potentially disastrous, as in the destruction of the Naiba River stock of chum salmon, and the reduced success of the inter-stock crosses of Atlantic salmon. Many stockings have failed, probably through lack of appropriate adaptation of the introduced stocks. At low levels of straying, barriers exist to effective reproduction of these strays. From this, and the discovery of sympatric non-interbreeding stocks , it is possible that some stockings can succeed, provided that the breeding behaviour and occupancy of the river environment by the introduced fish complement but do not interfere with those of the native population.
Behavioural differences exist between stocks.... As the developmental programme for organisms is genetically fixed but environmentally controlled, the performance of salmon transferred into a new habitat will depend on the nature of both the fish and the habitat. The same genetic stock of Atlantic salmon reared in two different environments in Canada matured at different ages in those tow environments. Conversely, in one environment in Ireland, the progeny of both grilse and 2 sea-winter salmon matured predominately as grilse. Thus the life-history pattern adopted by a successfully stocked fish would be heavily dependent on local conditions.
Consequently, the ideal material for stocking a system is that which is native to it. Since each salmon stock consists of diverse components, augmentation should aim to increase all those components in proportion to their occurrence in the initial population. To reduce the possibility of impairment of genetic diversity by inbreeding, at least 30 males and 30 females should be used as the founding broodstock of each stock component. In stocking systems empty of salmon, neighboring stocks are the most likely to provide successful material. In such cases there may be advantage in using several stocks initially, to broaden the genetic range on which selection may act.