Dynamics of an Atlantic salmon stock (Salmo salar) in a small Newfoundland river

Chadwick, E. M. P.(Edward Michael Pakenham) (1982) Dynamics of an Atlantic salmon stock (Salmo salar) in a small Newfoundland river. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Dynamics of an Atlantic salmon stock (Salmo salar L.) were studied in a small Newfoundland river, Western Arm Brook. The study examined dynamics of smolts, parr and adults. Smolt production was influenced by annual variation in year-class strength, smolt age, sex ratio and size. Year-class strength was significantly correlated with egg deposition. This was the first stock-recruitment relationship to be developed for Atlantic salmon. Supportive evidence was found on two other Newfoundland rivers, Indian and Little Codroy. On Little Codroy River, year-class strength of smolts was correlated (P < 0.01) with potential egg deposition of adults counted as kelts. On Indian River, egg to fry survival was correlated (P < 0.01) with winter temperature and discharge. On Western Arm Brook, smolt age was significantly correlated (P < 0.01) with annual mean monthly air temperature. Evidence was presented for density-dependent influence on both smolt age and sex ratio. Size of smolts had the lowest annual variation of all biological characteristics. Fork length, weight, ovarian weight and especially annual instantaneous growth rates of smolt were significantly (P < 0.01) different between smolt ages. Ovarian weight of smolts was inversely correlated (P = 0.01) to sea age of adult salmon in 34 Newfoundland rivers. -- Biological characteristics of parr were significantly different between the four habit types: steadies, riffles, outflows and lakes. Parr from riffles were smaller and younger. Parr in outflows grew most during the summer season. However, parr did not remain within habitats and there was a net downstream movement. Downstream movement of parr was significantly correlated with the size of the smolt run in the same year. Mean production in lake and steady habitats was 0.07 g m⁻² y⁻¹, and it was 2.23 g m⁻¹ y⁻¹ in riffles and outflows. Maximum production was estimated to be 5.47 g m⁻² y⁻¹. Only 33% of smolts were produced in riffles and outflows; the remainder were produced in lakes and steadies which comprised 98.6% of habitat accessible to salmon. Production was correlated with standing stock and over 50% was contributed by the second and third age groups. -- This paper presented the first evidence that a commercial fishery selected larger and older 1SW salmon. 1SW salmon spend one year at sea before first spawning. Grilse taken in the local fishery of St. Barbe Bay were significantly (P < 0.01) greater in fork length, whole weight, condition and smolt age than grilse entering the river. Selection for older smolt ages was due to a significant correlation between size and smolt age. The fishery also selected a greater proportion of repeat spawners and almost all 2SW salmon. Consequently fish which spawned were smaller and younger than in unexploited populations. There was also a considerable loss of iteroparity as a result of exploitation. -- A model was proposed to describe Atlantic salmon stocks in exploited and unexploited states. The model was based on density-dependent growth in freshwater. At low stock densities, salmon parr grew faster and went to sea at younger smolt ages. Faster growth induced precocity in male parr and shifted the sex ratio of the smolt migration to be predominantly female. At carrying capacity, smolt ages increased and smolt production was stabilized due to overlapping of year-classes. The economic benefit of increased stream biomass was a stable yield to the fisheries. The model was compared to trends in the commercial fisheries which included a 40 yr cycle of abundance, and declines in sea age and smolt age. A significant correlation between stock abundance and smolt age corroborated the proposed biomass model.

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