Ž .
Turnbull, 1992 . Most importantly, scanning electron micrographs of damaged fins from fish-farms showed clear tooth marks and an absence of bacterial infection
Ž .
Turnbull et al., 1996 . Fin damage has been used as an indicator of the strength of the social hierarchy by
Ž .
Ž .
Christiansen and Jobling 1990 and Moutou et al. 1998 , providing useful insights into the dynamics of aggression within larger groups of fish than can easily be studied
otherwise. Fin splitting is the primary symptom of fin damage, and repeated splitting eventually leads to tissue loss and thickening of the remaining tissue. Splitting heals
Ž rapidly, whereas re-growth and reduction in thickening take longer to occur Turnbull,
. 1992 . Therefore, splitting is likely to be the best indicator of current levels of
aggression. The aim of this study is to examine the effect of body size on the incidence of fin
damage in large groups of fish kept under culture conditions. Using data from individu- ally marked fish previously subjected to different manipulations of growth rates, we are
Ž .
able to compare the effects of both relative to other group members and absolute body size on the timing and duration of fin damage. We demonstrate a strong and consistent
effect of relative body size, which indicates the existence of alternative strategies of aggression and feeding within groups of fish.
2. Materials and methods
The experiments involved a population of farmed Atlantic salmon parr from pooled hatchery stock. The experiment started approximately 2 weeks after first-feeding, on 17
Ž .
April 1997, at the freshwater hatchery of Marine Harvest McConnell MHM
at Ž
. Invergarry, Scotland, where four tanks labelled A–D were stocked with 1550 7
fish per tank. In order to manipulate growth rates, group D, the control, remained at Invergarry throughout the experiment, while groups A, B and C successively spent 3
Ž .
Ž weeks in colder water mean of 8.3 0.028C in Glasgow University’s aquaria A from
. 17 April–8 May, B from 9–29 May and C from 30 May–19 June before being returned
to Invergarry. The water at Invergarry was heated to ca. 128C until mid-May, when ambient temperatures reached that level. The fish were then kept at the ambient water
temperature until the third week of October, when the water was heated to keep
Ž temperatures at ca. 88C. The fish were initially kept in small, circular tanks diameter 0.6
. m, water depth 0.25–0.3 m , but on 20th June all four groups, now permanently at
Ž .
Invergarry, were transferred to larger 2-m square tanks water depth 0.5 m , where they remained until the end of the experiment.
Throughout the experiment the tanks were lit by overhead fluorescent strip-lights; the photoperiod regime was that used commercially to produce accelerated ‘‘S1r2’’ smolts,
Ž .
with long days 24L:0D until the end of May, and thereafter 22L:2D separated by a Ž
. Ž
. photoperiod ‘‘winter’’ 10L:14D in the real early autumn. The fish were fed to excess
throughout the experiment on a pelleted diet of an appropriate size according to manufacturer’s tables. Food was dispensed from clockwork belt feeders in the small,
circular tanks and from hoppers in the large square tanks. Populations were thinned periodically when fish were removed for use in experiments to be reported elsewhere.
A random sample of 150 fish was measured on 18 April, the first day of the experiment. The fish were anaesthetised using Benzocaine and measurements were made
Ž .
of fork length to 1 mm . Random samples of 150 fish per group were then measured on 9 May, 29–30 May and 19–20 June. On 22–25 July, random samples of 100 fish per
Ž .
tank were measured and tagged with Passive Integrated Transponder PIT tags. The PIT Ž
. tags length 17 mm, mass 0.1 g were inserted into the body cavity through an incision
made in the body wall. The incision was then dusted with a 50:50 mix of Cicatrine Ž
. Ž
antibiotic powder Wellcome, London, UK and Orahesivee Protective Powder ER .
Squibb and Sons, Hounslow, UK to help prevent infection and close the wound. The tagged fish were re-measured on 1–3 September, 3–9 November and 9 December. From
July onwards only the tagged fish were assessed for fin damage, and data are presented only for the tagged fish that survived to the end of the experiment and were assessed on
Ž .
all sampling dates n s 314 . Thus, the changes in frequency should represent actual healing or incurring of damage rather than sampling error.
On each of the measurement days, damage to the dorsal fin was assessed by Ž
comparison with Fig. 1. Three separate measures were assessed: tissue loss judged by .
Ž .
fin size , splitting and thickening. Fin size was scored on a five-point scale Fig. 1 . As it was difficult to distinguish whether a fin was intact or only slightly reduced in size,
the 90 q category was taken to be undamaged and the lower categories were classed Ž .
Ž . as damaged. Splitting and thickening were both classed as either absent 0 , mild 1 or
Ž . severe 2 . Fin splitting categories 1 and 2 were combined for analysis as the distinction
between the two was judged in retrospect to be unsatisfactory.
Fig. 1. System used to score three categories of dorsal fin damage in juvenile Atlantic salmon.
Specific Growth Rate in length between measurement periods was calculated as: SGR s 100 = ln FL
y ln FL r t y t
Ž .
Ž .
Ž .
t 2 t1
2 1
Ž .
where t s first sampling day; t s second sampling day and FL s fork length mm .
1 2
3. Results
