W2A-027 Galway Bay sole-0.txt
Introduction

The thickback sole Microchirus variegatus (Donovan, 1808) is a relatively small species of the sole family, found on sandy and muddy ground. While recorded as deep as 400m (Quero et al. 1986), it is most abundant in depths between 37m and 92m (Wheeler 1969). Its distribution along the eastern Atlantic extends southward from north of the British Isles. It is also found in the Mediterranean and Adriatic, while its southern distribution is unknown (Quero et al. 1986). Rae (1970) gives an account of its distribution in Scottish waters.

Owing to its relative scarcity and small size, the thickback sole is of no economic importance, and hence very little is known about its general biology. Brief details on its rate of growth were given by Cunningham (1891-2), and Russell (1976) summarised the information available on the eggs and planktonic stages. Lebour (1918; 1920) gave an account of the four of the larval and young stages, whilst the feeding habits of the adults have been investigated by Stevens (1930), Sorbe (1972) and Quiniou (1978).

Little is known of its biology in Irish waters. Went and Kennedy (1976) and Dunne and Konnecker (1976) noted its occurrence, whilst Fives (1970) gave the seasonal abundance and distribution of the larval stages in the plankton off the west coast of Ireland. The present study was undertaken in order to increase our knowledge of the biology of the thickback sole in the Galway Bay area.

Sampling methods

The fish were caught incidentally by commercial vessels fishing for prawns (Nephrops norvegicus L.) and/or whitefish on sandy ground off the north shore of Galway Bay in the vicinity of Spiddal, west coast of Ireland (53 10' N, 9 47' W). A 70-80mm (stretched) cod-end was used. The thickback sole were collected by hand from extensive heaps of 'trash fish' on board these vessels. Prior examination in the laboratory, the fish were preserved by deep-freezing at -18 C.

Laboratory methods

(a) Age and growth.

Each fish was sexed, measured to the nearest millimetre below, and weighed without surface drying to the nearest 0.1g. Both standard (s.l.) and total (t.l.) length measurements were taken. The sagittal otoliths were removed and read following burning over a low gas flame for 10-30 seconds (Christensen 1964). During this process, the overall colour of the otolith changed form white to yellow-brown to black and finally ash grey. When examined microscopically, a clear pattern of alternating white (opaque prior to burning) and black (transparent prior to burning) zones could be seen on the surfaces of the fractured otolith. Age was determined by counting the number of white zones (i.e. summers of life), on the assumption that a white and black zone are laid down annually. January 1 was taken as the official birth date (Williams and Bedford 1974). A von Bertalanffy growth curve was fitted to the age-length data for each sex. The latter may be described by the equation:
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where Lt = length at time t, L = maximum theoretical length, K = growth constant and to = theoretical age at zero length. These parameters were calculated with the aid of the Fortran BGC 2 program (Abramson 1971), which was modified to suit the data.

(b) Reproduction.

The gonads were removed and weighed (wet weight) to the nearest 0.1mg. Maturity was assessed according to an arbitrary system whereby five stages were recognised for each sex, i.e. I immature virgins, II maturing virgins or recovering spents, III ripening, IV rope, V spent. The seasonal changes in gonad development were followed by calculating the monthly gonosomatic index, where GSI = gonad wt/whole body wt x 100 (Htun Han 1978). Data from equivalent months from different years were combined. Fecundity of the thickback sole was estimated by counting the number of ripening and ripe oocytes in a weighed portion of the ovary. The total number of oocytes in both ovaries was then calculated.

(c) Food.

The gut was removed, weighed (wet weight) and its contents identified. The diet was assessed using three methods of analysis, defined as follows. (i) Percentage points - Depending on the amount of food present, each gut was allotted 0-20 points. The number of points awarded to a food category was then expressed as a percentage of the total number of points allotted to the sample. (ii) Percentage occurrence - The occurrence of individual food items was expressed as a percentage of the number of fish in the sample containing food. (iii) Percentage number - The number of occurrences of a particular food category was expressed as a percentage of the total number of occurrences of all food items in the sample.

Results and discussion

Age and growth

A total of 512 thickback sole were collected between March 1978 and September 1979 in the Galway Bay area. Of this total 93 (18%) were male and 419 (82%) were female. The overwhelming predominance of females in the catches is attributed to the fact that the larger females were more frequently caught by the coarse meshes used. The study of thickback sole population growth and dynamics is thus limited in its overall observations.

The largest fish recorded was a fourteen-year-old female measuring 15.5cm (17.4cm t.l.), while the oldest was a seventeen-year-old female measuring 13.5cm (15.8cm t.l.). The largest male was a twelve-year-old of 13.0cm (15.2cm t.l.). Fish of similar length have also been recorded for this area by Dunne and Konnecker (1976). However, fish ranging in size from 21.5-22.5cm have been recorded elsewhere (see Cunningham 1891-2 for the Plymouth area; Rae 1970 for Scottish waters; Holt and Byrne 1905 for British and Irish waters; and Quiniou 1978 for Douarnenez Bay on the west coast of Brittany).

The length frequency distribution for (A) males and (B) females in the sampled population and in each age group (3 +, 4 +, 5 + etc.) is given in Fig. 1.A considerable overlap in length with age exists, which confounds any attempt to separate age classes by length for this species. Subsequent examination of the otoliths by burning (Christensen 1964) showed that the largest size grouping included fish with ages ranging from 12 to 16 years for males and from 8 to 17 years for females.

The otoliths of M. variegatus are typically sole-like in appearance. They are quite thick with a relatively rounded outline, plano-convex in section and, during the present investigation, ranged in size from 2.05-3.65mm. The otoliths were burned over a low flame; when the burned fractions were viewed under a dissecting microscope, alternating white and black zones could be clearly seen on the surface of the broken section of otolith. Figure 2 illustrates the seasonal variation in the percentage occurrence of a white or black zone at the otolith edge during the period of the investigation. Data from equivalent months from the two years have been combined. The zones were considered to be annular in formation. Maximum occurrence (95-100%) of a white (opaque prior to burning) zone at the otolith edge, indicating a period of increased growth, was recorded between May and August, while black (transparent prior to burning) zones were correspondingly less frequent during this time.

The mean length for each age group (95% confidence limits) is given in Fig. 3, together with the fitted von Bertalanffy growth in length curves. In order to remove the effect of a sampling bias, fish less than age group 5 for females and age group 6 for males were not included when estimating the parameters.

Growth is rapid for the first seven or eight years of life, after which it gradually slows down, with very little growth occurring after the tenth or eleventh year. Female fish are larger than males of the same age group. This difference, however, is not significant (p> 0.05).

A considerably faster rate of growth for M. variegatus was recorded by Cunningham (1891-2) at Plymouth where the fish reached 15.2cm in their second year. Cunningham, however, looked at the surface of unbroken otoliths. During the present investigation it was found that because of the thickness of the otolith an accurate age for the fish could not be calculated merely by viewing the otolith as a whole under the microscope. A maximum of about five rings only could be seen using this method. This became more obvious in the case of the larger and older specimens. Only after burning the otoliths (Christensen 1964) and examining the broken fractures could the age of the fish be determined with accuracy.

At Galway Bay females matured and spawned when they were three years old, having attained at least 9.0cm (s.l.) in length. Males, however, did not spawn until they had reached four years of age. The smallest ripe male recorded measured 8.2cm (s.l.). This difference in the age and length for first spawning is probably due to the slightly faster growth rate of the female specimens.

Figure 4 illustrates the percentage age composition of the sampled population. Fish less than 7.0cm (s.l.) in length and three years of age were not recorded during the present investigation. As the larval and young stages do not appear to differ in bathymetric distribution from the adults (Holt and Byrne 1905), their absence is no doubt due to the sampling bias in favour of the larger fish. The 1975 year class was the youngest group sampled efficiently by the sampling gear and was by far the most abundant cohort recorded in the catches. There was a steady decline in the numbers of fish surviving with time (1962-74), with no major recruitment from year to year. This pattern seems to indicate a relatively steady mortality rate for the thickback sole in the Galway Bay area.

Reproduction

It was not always possible to sex the fish externally. During the breeding season, however, females were easily distinguishable from males by the presence of large orange/red ovaries, the ventral one of which could be seen through the pale underside of the body. Very few immature (stage I) fish were recorded during the sampling period. Stage II fish were noted throughout the year. Ripening (stage III) fish occurred between January and June, whilst ripe (stage IV) fish were recorded from March to August. Spent fish (stage V) appeared in the catches in May and reached peak occurrence in September, when the spawning season came to an end.

The seasonal changes in gonad development during the study period were followed separately for both male and female fish, and the mean GSI values per month were calculated. Data from equivalent months for the two years were combined and the results are presented in Fig.5. The testes varied little in weight from month to month; however, a gradual increase in their weight was noted from March onwards. A slight peak was recorded in May, June and July when ripe males were abundant in the catches, while a decrease in value by August was indicative of an increase in the number of spent males. The female GSI values followed a similar pattern to that of the male fish. There was much more of an increase in GSI values for females from April to June. From the graph, peak numbers of ripe fish were recorded in June, July and August. A rapid decline in values from September onwards corresponded to an increase in the number of spent females. Information obtained from staging the fish is therefore substantiated by plotting GSI values, the results indicating a spawning period from May to August for M. variegatus in the Galway Bay area.

An earlier spawning period for the thickback sole of April-May has been recorded by Russell (1976) for the English Channel and by Cunningham (1891-2) for fish in the Plymouth area. These fish, however, were captured at latitudes south of the Galway Bay area, where water temperatures are slightly higher (Anon. 1981), thus possibly explaining, at least in part, the difference in spawning times between the various locations.
