Compensatory Growth, Life-History Decisions and Welfare of Farmed Atlantic Salmon (Salmo salar L.) Parr

MacLean, Amanda (1999) Compensatory Growth, Life-History Decisions and Welfare of Farmed Atlantic Salmon (Salmo salar L.) Parr. PhD thesis, University of Glasgow.

Abstract

In nature, growth rates are rarely maximised but are instead optimised by natural selection. Compensatory growth, a common phenomenon in the animal kingdom, is evidence of this: when growth conditions improve after periods of poorer than normal growth, many organisms will grow at faster than normal rates. This allows them to achieve a desired body size despite unexpected setbacks in growth. This thesis investigates compensatory growth in the context of social interactions and life-history decisions in Atlantic salmon Salmo salar. The fish in the experiments were 0+ parr on an accelerated smolt regime, producing out-of-season smolts ca. 6 months in advance of the natural cycle. In the experiment described in Chapter 2, six groups of individually marked fish were subjected to periods of unseasonably low temperature during successive periods of three or six weeks in the spring. Their growth rates were compared to that of a control group that was not exposed to low temperature. Four out of the six experimental groups showed clear compensatory growth spurts when they were returned to warm water. The mean size of fish in the two groups that did not exhibit compensatory growth was close or equal to that of fish that had already compensated. Thus, fish only initiated a period of compensatory growth if they were below a target size threshold for the time of year. Chapter 3 examines the behavioural mechanisms behind compensatory growth responses. By automatically registering movements of fish that were individually identified with passive integrated transponder (PIT) tags, the exact feeding and activity patterns of individual fish within groups were recorded over a period of days. While on average growth-compensating fish did not spend more time feeding than controls, they were more aggressive; as a result dominant fish within each group gained more exclusive access to the feeding area during periods of compensatory growth. The extent to which compensatory growth could be achieved was therefore dependent on both the social status of the individual and the dominants' ability to monopolise the food patch. Sexual maturation is an increasing problem in aquaculture due to the use of accelerated smolt regimes, probably because rapid growth rates trigger maturation during a crucial decision period in the spring prior to spawning. The experiment described in Chapter 4 used three groups of parr, exposed to successive three-week periods of low temperature between April and June, in an attempt to reduce the incidence of sexual maturation. There was some evidence that a growth setback caused a reduced and delayed investment in gonads. However, contrary to expectations, the incidence of maturation did not differ between the experimental groups and a control group. Periods of compensatory growth after the experimental groups were returned to warm water may have negated the effects of the low temperature treatment on the decision to mature. In addition, the absence of seasonal cues due to the constant photoperiod may have resulted in a less strictly defined decision window. Sexual maturation and smolting are often considered to be mutually inhibitory processes. However, some mature parr make the decision to smolt and do so with varying degrees of success. The question of whether the fish could undergo the processes of maturation and smolting at the same time was investigated in chapter 5 by following the development of smolt characteristics (smolt coloration and sea-water adaptability) in sexually mature and immature parr.. Smolt characteristics were more developed in immature fish, but nevertheless the mature males did show signs of smolting, and larger mature males could adapt to sea water. I suggest that the inhibition of smolting by sexual maturation is a result of two processes: firstly, that mature parr often do not fulfil the necessary requirements to make the smolt decision; secondly, that in mature parr that do decide to smolt, androgens inhibit or delay the development of smolt characteristics, but do not entirely prevent smolting due to the delay between spawning and emigration. The development of dorsal fin damage, which is primarily caused by aggression, was followed in Chapter 6 in four groups of parr of different mean length. The probability of having fin damage was strongly related to relative body size within each group: the largest fish in a tank were up to six times more likely to have damaged fins than the smallest fish. Studies of small groups of salmonids have demonstrated that subordinates are the main recipients of fin damage, but the present study indicates that the reverse is true in larger groups. This may be because dominant fish compete aggressively amongst themselves and incur fin damage, while less aggressive individuals adopt alternative feeding strategies that reduce the risk of injury. Erosion of the operculae is often seen in cultured fish, but little is known about the causes of the condition. While vitamin deficiency has been implicated in some cases, fish that show none of the other symptoms of vitamin deficiency may still develop opercular erosion. Chapter 7 describes how the development of opercular erosion was recorded in four groups of parr of different mean length. Erosion of the operculae developed during the early summer but healed completely during the late summer and autumn. The larger fish in a tank were generally more likely than smaller fish to have eroded operculae. I suggest that the condition resulted from physical injury caused by aggressive interactions between fish, and that a shift in behaviour with increasing body size reduced the rate of attack on the operculae later in the year, allowing them to recover from injury. The final chapter brings together the findings and concepts of the previous chapters. The implications of the findings for aquaculture are also discussed.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Neil Metcalfe
• Keywords: Zoology, Aquatic sciences
• Date of Award: 1999
• Depositing User: Enlighten Team
• Unique ID: glathesis:1999-76210
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/76210