Le Cren (1973) suggested that the most likely population-regulating process in salmonid populations is density-dependent territorial behaviour. Thus, in trout (Salmo trutta L.) populations aggressive territorial behaviour was manifested in density-dependent mortality and the size of the adult stock was largely dependent on the area of suitable rearing ground for the fry (Le Cren, 1973). Ricker (1954) also concluded that in most fish populations compensatory mortality in the juvenile stages, rather than in the mature stock, was of greatest importance as a regulator of population size. The nature of such regulating mechanisms in juvenile populations is, therefore, of fundamental importance in the understanding of the dynamics of fish populations and the implementation of management policies.

This paper considers mortality rates of young Atlantic salmon (Salmo salar L.) in the upper catchment of the R. Wye over a two year period. The work forms part of a broader study of fish populations in the R. Wye

Craig Goch Field Surveys Group

Gee, Alun S.

Hemsworth, R. J.

Milner, N. J.

Slater, Frederick Maurice

Online Research @ Cardiff

Craig Goch Report No. 10   The Effect of Density on Mortality in Juvenile Atlantic Salmon (Salmo salar L.)  By  Alun S. Gee, N.J. Milner & R. J. Hemsworth  University of Wales Institute of Science & Technology Field Centre, Newbridge-on-Wye, Powys, Wales  Running page heading:  Salmon Mortality  Introduction  Le Cren (1973) suggested that the most likely population-regulating process in salmonid populations is density-dependent territorial behaviour.  Thus, in trout (Salmo trutta L.) populations aggressive territorial behaviour was manifested in density-dependent mortality and the size of the adult stock was largely dependent on the area of suitable rearing ground for the fry (Le Cren, 1973).  Ricker (1954) also concluded that in most fish populations compensatory mortality in the juvenile stages, rather than in the mature stock, was of greatest importance as a regulator of population size.  The nature of such regulating mechanisms in juvenile populations is, therefore, of fundamental importance in the understanding of the dynamics of fish populations and the implementation of management policies.  This paper considers mortality rates of young Atlantic salmon (Salmo salar L.) in the upper catchment of the R. Wye over a two year period.  The work forms part of a broader study of fish populations in the R. Wye.  Description of the Study Area  The R. Wye, which is 250 km. long and has a catchment area of 4183 km3, rises at Plynlimon (667 m. O.D.) in Powys, mid-Wales.  This study was confined to the upper catchment of the river which drains impermeable Ordovician and Silurian sediments, principally mudstones and shales (Fig. 1).  Over the period of the study (February 1975 – November 1976) flows in the R. Wye were the lowest since records began (A. Tillotson, pers. comm.; Wright, 1976).  Records from a gauging station on the lower Wye show that for the 18 month period from April 1975 flows were 60% of the long term average (L.T.A.). The range during the study period was very large being 2% of the L.T.A. in June, July and August 1976 and 218% in October 1976.  Of the 16 study sites, five were located on the main R. Wye and the remainder on tributaries of the Wye (Fig. 1, Table 1).  These sites are typical salmon nursery areas having a substrate of mixed coarse sand and gravel.  Main river sites were predominantly riffle areas but tributary sites generally comprised a series of riffles and pools.  Average widths of the 50m long study reaches ranged from 10.5 to 26.0m. on the main river and -1- from 4.0 to 6.5m. on the tributaries:  the R. Elan (E1), a tributary receiving ‘compensation’ discharge from an impoundment (Fig. 1) was consistently 29m. wide (Table 1).  Methods Quantitative sampling, using D.C. electrofishing apparatus based on the design of Moore (1968), was generally undertaken four times a year during 1975 and 1976 at all sites except W4 and W5.  These sites were sampled in 1976 only.  At each site a 50m. reach was enclosed by stop-nets and fished three or four times.  Fork lengths of all salmon were measured and scales were taken for ageing purposes.  Where possible the catch-depletion data were used to make abundance estimates for each size-age group using the method of Zippin (1956).  Mortality rates were calculated from abundance estimates at successive sampling dates.  Results Mortality rates of 1975 year-class salmon only are considered because of the limited data available for other age-classes.  June was the earliest time at which fry were large enough to sample efficiently.  Within each site the mortality rate remained remarkably constant over the period June 1975 to November 1976 (Fig. 2).  Estimated abundance at each site declined exponentially with respect to time according to the standard expression:    Nt = No exp (-Zt)        (1)  where  Nt = number present at time t (days)     No = number present at beginning of the period (t = o)   Z  = exponential rate of decrease or mortality.  Using the method of at least squares regression the line of best fit was calculated and Z estimated for each site (Fig. 2).  It was then possible to estimate the numbers of salmon fry (No) at each site at a common time, 1 June 1975 (to), and hence, knowing the area (Ao) of each site at this time, fry density (No/Ao).  The relationship between the instantaneous rate of mortality, Z, over the 18-month period and the density  of fry on 1 June 1975 is significant at the 0.001 level (r – 0.827, d.f. = 15) (Fig. 3).  If sites W4 and E1, which were substantially larger than the other sites (Table 1) are excluded from the analysis the relationship (fitted by the least squares method) is best expressed by (Fig. 3):-    Z = 0.0026 + 0.0021 (No/Ao)      (2)  Combining equations (1) and (2) the numbers of 1975 year-class salmon (Nt) surviving to time t can be calculated from:    Nt = No exp (-(0.0026 + 0.0021 (No))t)                                                 (Ao)) )     (3)  Assuming a fry density on 1 June 1975 and using 1 March as a convenient birthday for salmon it is possible to estimate, using Equation (3), the number of salmon surviving until the second birthday (March 1977) (Fig. 4).  Since most 2 year-old salmon (parr) in the -2- Wye are in fact pre-smolts at this time (Hutton, 1937) their abundance can be considered a good estimate of smolt production for the area during the year.  It is clear that optimum smolt production in the upper Wye catchment results from fry densities (1 June) in the region of 0.75m-2 (Fig. 4).  Of the sites sampled, three (W2, M1 and D3) have fry densities likely to produce maximum smolt production, three (W1, M2 and B3) have fry densities too high (Fig. 4).  The range of expected smolt densities is 0.005 – 0.052m-2, with 85% of the sites producing over 0.025 smolts m-2.  Discussion Juvenile salmon and trout are strongly territorial (Kalleberg, 1958; Le Cren, 1973).  The expected relationships between the mortality rate and the initial density would, therefore, be semi-logarithmic; the mortality rate would be asymptotic at high initial densities (see Fig. 3), as found for trout by Le Cren (1973).  However, in this study this was not the case.  Above initial densities of 0.75m-2 the relationship between mortality and density is over-compensatory (Fig. 3) such that the number of survivors decreases as density increases above the optimum density.  In this study and Le Cren’s (1973) experiments, post-June mortality rates within each site were constant and a function of the June density.  It appears, therefore, that initial high densities (> 0.75 m-2) confer some disadvantages even to those individuals that survive the initial period of higher mortality.  This disadvantage was not manifested in growth rates, which followed the normal pattern of being inversely density-dependent (unpublished data).  There may well be stress effects, resulting from the initial high density conditions, which continue to operate even at lower densities.  Miller (1958) found that transplanted hatchery-reared trout accumulated high blood lactate levels and died either from acidosis or starvation as a result of aggression in competition with native trout.  A similar decrease in fitness may occur in young salmon at high densities as a result of an increase in the number of intra-specific encounters.  In contrast to the observations of Le Cren (1973) on brown trout, variations in the abundance of coexisting species both within and between studies over the period of study (Table 1) did not significantly affect mortality rates (Fig. 2): this is presumably because changes in such wetted areas are much lower than the differences in population densities between sites.  This is of relevant importance in interpreting the effects of changes in wetted areas, whether natural or artificial, on the survivorship of salmon populations.  Data from the two sites, W4 and E1, were not in close agreement with those collected at other sites and it may be significant that the river at these two sites is considerably wider (approx. 100%) than any of the other sites.  Clearly, there are difficulties in adequately sampling such wide river sites which may have resulted in biased abundance estimates.  Alternatively, the general density-dependent relationship (Fig. 3 Equation [2]) may not be the best description of the relationships pertaining at these sites, although data from a nearby site W5, fitted the relationship well.  This is evidence that under favourable conditions the mortality of salmonid eggs and alevins is low and that high mortality does not occur until the fry stage (Hobbs, 1940; Allen, 1951; Jones 1959).  In the absence of destruction of previously laid eggs by later spawners (‘overcutting’) fry density measured on 1 June, is, therefore, likely to be proportional to spawning intensity.  Since variation in adult mortality is small compared -3- with that in the juvenile stages and less likely to affect recruitment (Ricker, 1954; Le Cren, 1965).  The model depicted in Fig. 4 can be regarded as a reproduction or stock-recruitment curve and is similar to observed curves for non-territorial species, including Oncorhynchus gorbusha Walbaum (Ricker, 1954) and Melangrammus neglifinus L. (Cushing & Harris, 1973).  Expected smolt densities in the Wye catchment calculated from Equation (3) are similar to density estimates from migrating smolts caught in traps in other rivers (Elson, 1957; Mills, 1964; Meister, 1962; Jessop, 1975).  Many sites on the R. Wye appear to sustain fry densities which are too high for optimum smolt production (Fig. 4).  More work of the kind done in North American rivers (Elson, 1957; Wickett, 1958; Meister, 1962) is needed before salmon stocks in Britain can be managed by manipulation of the spawning escapements.  The more immediate fishery management implications are that, assuming hatchery reared fish behave as native fry, the optimum stocking density should be such as to produce about 0.75 fry m-2, considerably lower than the density of 2-5m-2 recommended for moorland streams by Mills (1969).  Acknowledgements This work formed part of a contract sponsored by the Welsh National Water Development Authority and supported by funds from Severn-Trent Water Authority and Central Water Planning Unit.  The authors wish to thank the Llysdinam Trust and numerous land and fishery owners for their co-operation, and Dr. M.P. Brooker for his advice and assistance.  Professor R.W. Edwards read and commented upon the manuscript.  Summary 1. The mortality of juvenile Atlantic salmon (Salmo salar L.) was investigated in 16 study sites in the upper Wye catchment during the period June 1975 to November 1976.  2. Within each site the instantaneous mortality rate remained remarkably constant over the period of study and the mortality rate at each site was directly related to the density of fry on 1 June.  Thus, knowing the fry density on 1 June it was possible to predict the survivorship of juvenile salmon.  3. The number of survivors decreased above the optimum initial density as indicated by a dome-shaped recruitment curve atypical of territorial species.  4. Assuming that smolt production is equivalent to the numbers of juvenile salmon surviving to 2 years of age, the maximum smolt production of 0.052m-2 is estimated to be attained from a fry density on 1 June of 0.75m-2.  Many sites on the R. Wye appear to sustain fry densities which are too high for maximum smolt production.  The optimum stocking density for unfed fry is 1m-2, considerably lower than is common practice.     -4- References Allen, K.R. (1951).  The Horokiwi Stream: A study of a trout population.  Fish. Bull. N.Z. 10, 1-238.  Cushing, D.H. & Harris, J.G.K. (1973).  Stock and recruitment and the problem of density dependence.  Rap. P.-V Reun. Cons. Int. Explor. Mer., 164, 142-155.  Elson, P.F. (1957).  Number of salmon needed to maintain stocks, Can. Fish Culturist, 21, 19-23.  Hobbs, D.F. (1940).  Natural reproduction of trout in New Zealand and its relation to density of populations.  Fish bull. N.Z. 8, 93pp.  Hutton, J.A. (1937).  Wye parr and scolts: the inverse-ratio theory of river and sea life.  Salmon and Trout Magazine. 87, 19-123.  Jessop, B.M. (1975).  Investigation of the salmon (Salmo salar) smolt migration of The Big Salmon River, New Brunswick, 1966-1972.  Fisheries Development Branch, Fisheries and Marine Services Department of the Environment.  Tech. Rep. Serv. No. MAR/T-75-1, 57pp.  Jones, J.W. (1959). The Salmon.  Collins, London.  Kalleberg, H. (1958).  Observations in a stream tank of territoriality and competition in juvenile salmon and trout (Salmo salar L.) and S. trutta L.).  Rep. Inst. Freshwat. Res. Drottningholm, 39, 55-98.  Le Cren, E.D. (1965).  Some factors regulating the size of populations of freshwater fish.  Mitt, Internat. Verein. Limnol. 13, 88-105.  Le Cren, E.D. (1973).  The population dynamics of young trout (Salmo trutta) in relation to density and territorial behaviour.  Rep. P-V Reun. Cons. Int. Explor. Mer., 164, 241-246.  Meister, A.L. (1962).  Atlantic salmon production in Cove Brook, Maine.  Trans. Amer. Fish. Soc., 19, 208-212.  Miller, R. B. (1958).  The role of competition in the mortality of hatchery trout.  J. Fish Res. Bd. Can. 15, 27-45.  Mills, D.H. (1964).  The ecology of the yung stages of The Atlantic salmon in the River, Bran, Ross-shire.  Freshwat, Salm, Fish, Res. 32, 58pp.  Mills, D.H. (1969).  The survival of hatchery-reared salmon fry in some Scottish streams.  Freshwat. Salm. Fish. Res. 39, 12pp.  Moore, W.H. (1968).  A ligh-weight pulsed D.C. fish shocker.  J. Appl. Ecol. 5, 205-208.  Ricker, W.E. (1954).  Stock and Recruitment.  J. Fish. Res. Bd. Can. 11, 559-623.  -5- Wickett, W.P. (1958).  Review of certain environmental factors affecting the production of pink and chum salmon.  J. Fish. Res. Bd. Can. 15, 1103-1126.  Wright, C.E. (1976).  Once in 1,000 years?  Water, November, 2-6.  Zippin, C. (1956).  An evaluation of the removal method of estimating animal populations.  Biometrics, 12, 163-189.    Table 1  Physical dimensions of Sampling Sites  Area m-2 Average depth, cm. Station June 1975 Range 1975/76 June 1975 Range 1975/76D1 260 216-330 26 19-32 D2 315 300-368 25 22-40 D3 240 240-311 18 16-26 B1 195 189-267 14 9-37 B2 195 172-330 23 23-44 B3 200 157-230 27 13-31 M1 310 297-496 16 11-38 M2 310 277-446 21 15-49 C1 240 162-354 24 12-31 C2 255 255-379 20 14-26 W1 538 408-591 24 13-27 W2 633 575-948 25 21-37 W3 500 500-652 33 33-43 W4 1270 635-1270 32 16-58 W5 627 612-701 31 24-35 E1 1149 - 36 - -6- Figure Legends  Fig. 1.  Map of the study area to show the sampling sites.  Fig. 2.  Mortality curves for the 1975 year-class salmon in 16 study sites indicating   the magnitude of the instantaneous mortality rate (per day) Z.  Fig. 3.  The relationship between the instantaneous mortality rate (Z) and fry density   as estimated for each site from the mortality curves of Fig. 2.  the ‘line of    density-dependence’ indicates the relationship between Z and fry density    necessary to produce asymptotic recruitment.  Fig. 4.  Expected ‘smolt’ production at varying fry densities as predicted from    equation (3).  The observed densities of fry on 1 June 1975 for sites in the   Wye catchment are  indicated.  -7- Figure 1BidnoB3B2B1W1W2MartegM2M1W3Elan Valley ReservoirsRhayaderW4W5ElanE1EithonChwefruBuilth WellsC2 C1Irfon DuhonwD3D2D1WyeEdwN0                                                               10KMFigure 2D1z = 0.0104r = 0.9324104103102101    2     3    4    5     6     7 1    2     3    4    5     6     7 1    2     3    4    5     6     7 1    2     3    4    5     6     7D2 D3 B110310210z = 0.0076r = 0.977910310210110310210z = 0.0041r = 0.9941z = 0.0071r = 0.9967B2 B3 M1 M21    2     3    4    5     6     7 1    2     3    4    5     6     7 1    2     3    4    5     6    7 1    2     3    4    5     6    7103102101031021010310210z = 0.0052r = 0.9782z = 0.0039r = 0.9013z = 0.0025r = 0.9248z = 0.0023r = 0.9225Salmon per 50m length of stream103102101Figure 2 cont.C1z = 0.0069r = 0.97741    2     3    4    5     6     7Salmon per 50m length of streamE1W5W4W3W2W1C21    2     3    4    5     6     71    2     3    4    5     6     71    2     3    4    5     6     71    2     3    4    5     6     71    2     3    4    5     6     71    2     3    4    5     6     71    2     3    4    5     6     7z = 0.0076r = 0.9161z = 0.0035r = 0.9999z = 0.0032r = 0.9924z = 0.0080r = 0.9905z = 0.0071r = 0.9977z = 0.0104r = 0.9769z = 0.0078r = 0.9942103102101031021011021011031021010310210103102101031021010310210Time from 1 March 1975 (days x 10-2)Figure 3Instantaneous mortality rate (per day) z     (x 103)1098765432101                                2            3                                4         Fry density on 1 June (m-2)D1W5W3D2C1B1C2E1W4B2D3B3W1 W2M1M2Line of density-dependencer = 0.9410Figure 4Fry density on 1 June (m-2)1                    2                    3                    4 50504030201Number of two year old parr m-2 on 1 MarchW5D1W3D2C1B1B2C2D3W2M1B3M2W1

Craig Goch Report No. 10

The effect of density on mortality in juvenile Atlantic salmon (Salmo salar L.)

https://orca.cardiff.ac.uk/id/eprint/52455/1/Craig%20Goch%20Report%2010.pdf

Craig Goch Report No. 10 The effect of density on mortality in juvenile Atlantic salmon (Salmo salar L.)

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