studied the recruitment of young herring in the Moray Firth to their adult stock,. Dragesund (1970) reviewed the work on recruitment of Norwegian spring- ...
I. exp. mar. Biol. Ecol., 1973, Vol. 12, pp. 119-144;@ North-Holland
Publishing Company
ABUNDANCE, STRUCTURE, GROWTH AND ORIGIN’OF INSHORE CLUPEID POPULATIONS
OF THE WEST COAST OF SCOTLAND
S. S. DE SILVA ’ The Dunstafnage Marine Research Laboratory, Oban, Argyll, Scotland and The University of Stirling, Stirling; Scotland Abstract: The abundance, structure, growth, and origin of clupeid populations in inshore waters of the west coast of Scotland were studied from April 1970 to October 1972. Clupeid populations in the area comprise mainfy young fish. O-group autumn-spawned herring, probably of Minch origin, move into the area about April and spring-spawned ones (Clyde origin) about June. The timing and the body length at which each group arrives in the area during the different years is the same. After the initial immigration, the distribution of both O-group clupeids becomes localized. Herring populations in the sea-lochs and associated inshore waters are mainly O-group fish, which are replaced each year by a new incoming brood. The sprat populations of the sea-lochs are dominated by the O-group; in the more ‘open’ areas the populations comprise older individuals. Year-class distribution in the ‘open’ areas resemble that of the commercial fishery. The rate of increase of O-group autumn-spawned herring is 3.68 mm/week and that of springspawned O-group herring is 2.83 mm/week. The curves of growth of O-group sprats of the 1970 and 1971 year-classes are different; the rate of increase in length, however, averages 3.55 mm/week for both year-classes and the differences are not significant. In sprats, after their first year of life a rapid increase in length takes place in the spring. This increase is thought to enable the majority of the population to reach the minimum size (88-90 mm) for initial gonadal maturation and thereby make them capable of reproducing in their second year of life.
INTRODUCTION The two
species of clupeids present on the west coast of Scotland are the herring commercially, herring have ranked among the three most important species in European waters, until recently research has been concentrated mostly on commercially viable aduh stocks, supported by regular larval surveys; much less is known about the biology of young herring. Consequently, this has left a gap in the understanding of certain changes that have been experienced in the fisheries. The taxonomy, geographical distribution and specific characters of the two clupeids are dealt with by Svetovidov (1952) and Wheeler (1969). In recent years, Saville (1971) studied the recruitment of young herring in the Moray Firth to their adult stock, Dragesund (1970) reviewed the work on recruitment of Norwegian spring-spawned herring, and Wood (1959, 1971) has discussed the origin and length frequency distriClupeu harengus L. and the sprat Sprattus spruttus (L.). Although,
1 Present address:Department of Zoology, Vidyalankara Campus, University of Ceylon, Kelaniya, Ceylon. 119
bution of whitebait herring in the southern North Sea. Robertson (1938) reviewed the sprat fisheries in England, but important aspects of the biology such as food, feeding and reproduction were hardly dealt with. Johnson (1970) studied the Wash sprat fishery from the point of view of commercial statistics, giving growth rates, size and age composition, minimum size of maturity, and some aspects of reproductive biology. On the Scottish west coast there has been a sprat fishery since November 1968 (Langham, 1969) and this is of special relevance to the present study since it is almost entirely confined to the sea-lochs. The biology of O-group sprats is afmost corn~~et~iy unknown except for the fact that their occurrence with O-group herring has been observed by a number of authors (Daniel, 1947: Bowers, 1952). The west highland sea-lochs are in some ways comparable to the Norwegian fjords which are the main nursery grounds for the Norwegian spring-spawned herring (Dmgesund, 1970) and which also support a profitable fishery for young herring during the winter months. in this paper an attempt is made to assess the importance of sea-lochs and inshore areas as nursery grounds for young clupeids. The relative abundance, population structure, growth, and origjn of clupeids have been studied.
Sea-lochs and associated inshore sea areas were sampled at regular monthly intervals, as far as possible, from April 1970 to October 1972. The main sampling grounds, and the type of gear used are shown in Fig. 1. The hydrography and bottom topog-
Fig. I. Map showing the areas of sampling and the gear used at each station; MT, mid-water trawl; BT, bottom trawl; BS, beach seine: Sfations I (MT) and 2 (MT, B’T, BS), Loch Etive; Stations 3 (MT, BT, BS) and 4 (MT, BT), Firth of Lorne; Station 5 (MT, BT), Loch Linnhe; Station 6 (MT, BT), Loch Spelve; Station 7 (BT), Loch Creran; Station 8 (MT, BT), Bloody Bay; Station 9 (MT, BT), Loch Sunart; Station IO (BT), Loch Scridain.
INSHORE
CLUPEID
POPULATIONS
121
raphy cf Lochs Etive and Creran are dealt by Gage (1972) and that of Loch Linnhe by Pearson (1970). Sampling in Loch Creran had to be discontinued after November 1970 because it was found to be unsuitable for ground trawling. Sampling in Loch Linnhe had to be confined to the lower reaches of the Loch, due to the lack of ship time. During the 1970-71 commercial sprat fishery three samples, each of 100 fish, from sea-lochs on the west of Mull were obtained. Though samples from the 1971-72 fishery were not obtained, Dr. R. Bailey of the Marine Laboratory in Aberdeen has provided data for the west coast fishery which will be used in this paper. Three main types of gear, two bottom trawls and one mid-water trawl were used from R. E’. Cahnus and on occasions a beach seine was used. Until September 1971, a Vigneron-Dahl, manilla bottom trawl was used alternately with a Gourock No. 1 mid-water trawl. The mid-water trawl was f&t used in June 1970 and thereafter every month until August 1972. The bottom trawl was replaced by a light wing-trawl in August 1971. The detailed specification of the gears and their efficiency are dealt with fully by De Silva (1973). TREATMENT OF MATERIAL
After the net had been hauled, herring and sprats were separated and counted. If the catch were too large the total was estimated by sub-sampling, SO-100 fish of each species being randomly taken for detailed analysis. Total and standard (from the tip of the snout to the base of the caudal fin) length of each fish were measured, the gonads and the gut removed, and the fish weighed to the nearest 0.1 g. A vertical transverse incision was made at the back of the skull to expose the largest pair of otoliths, the sagittae, which were carefully removed, washed, dried and stored and later mounted in resin on black plastic slides for examination (see Parrish & Sharman, 1960). Otoliths were examined under a binocular microscope and the number of hyaline zones counted to determine the age of individual fish. The length from the centre of the nucleus to each hyaline zone and the total length of the otolith was also measured using a micrometer eye-piece. 80-100 fish from each sample of herring and between 200 and 300 sprats from each locality were used for the determination of their meristic characters. In herring the characters that were counted were the number of vertebrae including the urostyle and the number of keeled scales between the pelvic fins and the anus (K,); in sprats these characters and the number of rays in the pectoral and pelvic fins were also determined. RELIABILITY OF ESTIMATES
The abundance estimates obtained from catch data are only relative estimates, since a number of types of gear were used and since the area of sampling in any given loch
122
S.S.DE SILVA
was seriously limited by shore and bottom topography. For example, along the entire length of Loch Etive (17 miles) only one region, Airds Bay, was suitable for bottom trawling. Night fishing using mid-water gear was restricted to three stations in Loch Etive, the Firth of Lorne and the Bloody Bay, where space was adequate. In Lochs Spelve and Creran the area for manoeuvring was a limiting factor even in daylight while in Loch Sunart the presence of small islands made it difficult to trawl. In the case of the O-group fish, more than 95 % of the catches were made with one type of gear, the mid-water trawl; for the other catches allowance has been made, as far as possible, for any change of gear (De Silva, 1973). RESULTS ABUNDANCE
The relative seasonal abundance of O-group herring and sprat of year-classes 1970 and 1971 are given in Fig. 2. Fig. 3 shows the relative abundance of herring and sprat in their first year of life and thereafter for each area. Year-class abundance of both
to-l,,,,,,, AMJ
_
1970 YEAR-CLASS
----
1971
,I
,.
#I
,,,,,
JASDN’DJFMAMJ
Fig. 2. Relative abundance of O-group herring and sprats of the 1970 and 1971 year classes (log-scale).
species at six-monthly intervals with respect to age is given in Fig. 4. In general, the abundance estimates for the 1971 year-class of both species were higher than those for the 1970 year-class and this is thought to be due to an increase in fishing efficiency as a result of increased experience with the gear rather than a difference in the strengths of this year-class. In 1970, O-group herring were not obtained in the catches until June, when the mid-water gear was used for the first time. The presence of herring in the gut of whiting and spur-dogfish (Dr J. D. M. Gordon, pers. comm.) indicates that herring were present in the area in April The 1971 and 1972 year-classes of
1NSHORE CLUPEID POPULATIONS
123
herring made their appearance in the catches in April so that there must have been an immigration of young herring into the area, starting about April. There is a progressive increase in the abundance of herring (Fig. 2) until about September, when numbers as high as 1700/h fishing are reached. During the following winter months there is a gradual decrease in abundance and by March-April of the following year the number of herring present in the area is very small. From Fig. 3 it is evident that the herring coming into the area are first found in the Firth of Lorne and then soon move into the sea-lochs. Loch Etive is the last to be ‘invaded’ probably because of local topography: the loch is narrow and has a sheltered entrance. After the initial movement into the lochs, young herring tend to move out of the smaller sea-lochs such as Spelve and Creran within two months, and are very rarely caught in them thereafter. The overall decrease in relative numbers in the winter months could be due to either mass mortality, emigration, or both. Predation is thought to be one of the main causes of mortality, the predators present being spur-dogfish, whiting, mackerel, and sea-trout; there is no evidence of an increase in the number of predators in the winter, nor of an increased dependence of these predators on young herring (Dr J. D. M. Gordon, pers. comm.). It is very likely that the decrease in abundance of herring during the winter is mainly due to an emigration, probably into deeper waters outside the area sampled. There is, however, no decrease in Loch Etive and large numbers are found there during the winter. It was also noted that these fish are mainly found in a trench 80 to 150 m deep which extends from the head of the lower loch into the upper loch. These O-group herring were found to originate from both autumn and spring-spawning races (see p. 137). Spring-spawned herring of the 1970, 1971, and 1972 year-classes first appeared in June at mean lengths of 44.5, 44.2 and 42.4 mm. The autumnspawned fish were caught in April 1971 and 1972 with mean lengths of 44.8 and 40.4 mm, respectively. Thus, herring both from different parental stocks and from the same stock of different years arrive in the area at a similar mean and range of lengths. The timing of the arrival is also the same for all year-classes within any one race. Unlike herring, O-group sprats move into the area at different times of the year. In 1970 they were first caught in August and in 1971 in June. They also show a similar pattern in abundance when the whole area is considered (Fig. 2) but are different from herring in their relative distribution. Unlike herring they tend to remain for a longer time in the small lochs. From September onwards there is a decrease in numbers in the area as a whole but concentrations of O-group sprats are also found in Loch Etive in winter. Fish were aged from the 1st January. From Fig. 3 it is evident that after May there is an almost complete absence of I+ and older herring in the area. There were sporadic increases in numbers of older herring in the Firth of Lorne in November 1970 and in Bloody Bay in November 1971 which are thought to be due to emigrating fish. The increase in numbers in Loch Etive from January to
134
S. S. DE SlLVA -----o
M
o- -a L.LINNHE .-- --a L.CRERAN
L. ETIVE L.SPELVE
h------d F.of LORNE L--- --A BLOODY BAY
:
b
Fig. 3. Relative
abundance of herring and sprats in each locality: A, O-group sprats; C, 1 1 and over herring: D, I -f and over sprats.
herring;
B, O-group
INSHORE
CLUPEID
POPULATIONS
125
April is due to the O-group herring which concentrated in the loch during winter, becoming 1 year old on January 1st. Older sprats are found in appreciable numbers throughout the year in the Firth of Lorne and Bloody Bay while in the sea-lochs mainly one-year-old fish and a few twoand three-year-olds are found in the spring. The general trends in abundance are an increase in the numbers of old fish during the winter months, a gradual decrease from March till June, a slight increase for one or two months, and then a decrease coincidental with the arrival of the new year’s brood (Fig. 3). The slight increase in the abundance of older sprats in the winter is probably a result of the immigration of over-wintering sprats (Robertson, 1938; Langham, 1969) and, from March onwards, migrations of mature fish to the spawning grounds and the tendency to move into deeper water with age would cause a decrease. The O-group fish will become one-yearold in January and this increase, together with the over-wintering older sprats would cause a significant increase in numbers at this time as shown in Fig. 3. POPULATION STRUCTURE
Year-class abundance
There is a gradual reduction in numbers with age in both species, the most significant being soon after the end of their first year of life (Fig. 4). This is probably due to the tendency for clupeids to move into deeper waters with increasing age as has -
1966 YEAR-CLASS 1967 *’ *a
-
1969 YEAR-CLASS 1970 I’ II
w
1968
-
1971
8’
AGE
Fig. 4. Abundance
81
”
*I
IN YEARS
of year classes of herring and sprats, at six monthly intervals with age.
126
S. S. DE S&VA
been noted by others (Parrish & Saville, 1965; Dragesund, 1970). Decrease in numbers with age is much less pronounced in sprats, as is evident from comparisons of abundance of corresponding year-&asses in the two species. ft is also evident that certain year-classes, for example the 1968 year-class, occur in greater abundance even at an older age and it is possible that this may be indicative of the year-class strength. This will be dealt with in detail in the following section.
In arriving at the age d~strjbut~on ofthe population, samples obtained in two consecutive months were pooled together. This was done to compensate for errors due to mesh selection and gear selectivity of any one type of gear: a combined sample, obtained from mid-water and bottom gear, was expected to give a better representation of the population (Ricker, 1958). As stated earlier (p. 123), because of the absence of herring in Lochs Spelve, Creran and Sunart and the similarity in age distribution in the Firth of tome, Moody Bay and Loch Etive, the age composition di~granls are based on pooled data from the latter three areas {Fig. 5). The age composition of herring is dominated by one year-
Fig. 5. Age distribution of the herring popuhtions from April 1970 to October 1972.
fish until about April-May when the appearance of the O-group and the absence of 1 + fish allows the O-group to be more dominant. Two- and three-year-olds constitute a very small proportion of the population and their occurrence is rather sporadic. The age cornpos~t~~n of sprat populations in Lochs Spelve, Etive. Creran and aIso those on the west of Mull (Loch Scridain) and in the ‘open’ areas, Firth of Corne and Bloody Bay, are given in Figs 6 and 7 respectively. For purposes ofcomparison of the sea-loch and ‘open’ area populations with that of the ~ornrn~~c~~~fishery, year-class
old
INSHORE
CLUPEID
POPULATIONS
127
compositions from April 197~March 1971 and April 1971~March 1972, were calculated separately for the sea-lochs and for the Firth of Lorne and Bloody Bay (Fig. 8). Sprat populations in the sea-lochs are dominated mainIy by one-year-old fish, which gradually become replaced by the O-group of every new year-class. There is complete replacement of the l-group fish in two-four months. Fish older than one year are present in a small proportion, and enter into the population during the winter months. After the winter there is a gradual decrease of 1 f and older fish and by July-August they constitute an almost insignificant proportion of the population. L.SPELVE
L..%RIDAIN
m
AGE IN YEARS Fig. 6. Age d~strjbution of the sprat populations in the sea-lochs Spelve, Creran, Etive and Scridain from April 1970 to October 1972.
In the winter months 2-, 3-, and 4-year-old fish enter Loch Spelve but not Loch Etive. In Loch Spelve an increase in the numbers of old fish is known to occur during the winter and this probably causes a change in age structure at this time. It is probable that because of the local topography of Loch Etive, these over-wintering old fish do not find their way into the loch. In contrast to the sea-lochs, there is no complete replacement of l-year and older fish by the O-group in the ‘open’ areas. These areas also differ in that a high proportion of fish aged 2, 3 and 4 and at times even 5-year-olds are present in the population
128
S. S. DE SILVA
AGE
Fig. 7. Age distribution
IN cVEARS
of the sprat populations
in the ‘open’ areas from April 1970 to October 1972.
YEAR - CLASS Fig. 8. Camparison
of the year ctass composition of sprat popufations in the sea-lochs, the Firth of Borne and Bloody Bay from April 197tSMarch 1971 and April 1971-March 1972 with that of the 1970-71 and 1971-72 commercial fishery statistics of the Scottish west coast.
INSHORE
CLUPEID
POPULATIONS
129
(Fig, 7). It is evident that the 1968 year-class was dominant as 2-year-old fish in 1970 and as 3-year-old in 1971, in the Firth of Lorne and Bloody Bay. When the age composition in the Firth of Lorne and Bloody Bay is compared with that of the commercial samples from the 1970-71 fishery, the similarity is apparent. The dominant year-class in the fishery was that of 1968, constituting more than 70 % of the catch. The Bloody Bay populations resemble those of the commercial fishery more closely than do the Firth of Lorne populations where the 1970 brood constitute a moderate proportion and so the age structure of the Firth of Lorne population is less similar to that of the commercial fishery. The year-class composition of the 1971-72 sprat fishery is shown in Fig. 8. Its similarity to the Bloody Bay pop~ations and also to that of the Firth of Lorne is very evident. The evidence suggests that in the ‘open’ areas the age structure or year-class composition does give an indication of the composition of the commercial fisheries. GROWTH
Growth of O-group herring
Growth of O-group herring of the 1970, 1971 and a part of the 1972 year-classes was determined from the mean length and weight of samples of SO-100 fish taken at regular intervals. The length-frequency distribution of each half centimetre length group in each sample was also determined. Meristic characters were determined for every sample of herring, on the basis of which they were allocated into either the spring or the autumn spawning race, or a mixture of the two. From the mean lengths and the length-frequency distributions of the samples in any one locality it was evident that the rate of growth of the different year-classes was similar. Increase in the mean length of O-group spring- and autumn-spawned herring is shown in Fig. 9, which indicates that growth in length of O-group herring continues until September-October but ceases during the winter months. Resumption of growth after the winter in autumn-spawned herring takes place in April-May. There was no observable year-class difference in the timing of the growth check nor in the resumption of growth after the winter. There is no information as to the time of resumption of growth in spring-spawned herring. The rate of growth was calculated for the two races in the different localities from the slope of the curve (Fig. 9) during the main growth period before the winter. The growth rate of autumn-spawned herring in Loch Etive and the Firth of Lorne was found to be 3.60 mm/week and 3.30 mm/week, respectively. The growth rates of spring-spawned herring were much lower, with increases of 2.87 mm/week and 2.78 mm/week in Loch Etive and the Firth of Lorne respectively. The observed differences in the growth rates within each race were not statistically significant, but the differences in growth rate between the two races were significant (P < 0.01). The size-frequency distribution of both groups of herring in each locality is given in Fig. 10. Since there was no apparent difference in the rate of growth of year-classes,
S. S. DE SILVA
130
samples from different year-classes were pooled together when they were obtained within the same week of each year. It is evident from Fig. 10 that there is a gradual increase in length up to October and also that, in generaI, the samples were homogeneous and that the size range within a sample did not increase much as the year progressed. It is evident that the first winter ring in autumn-spawned herring in this area is formed at mean length of 12Sf28 mm and that of s~ring~spawned herring at S-90 mm. These values are in agreement with those observed from back calculations from otoliths in one-year-old fish of the respective races. The mean rate of growth of autumn-spawned herring (3.46 mm/week) in this area is in good agreement with the ..-.+ 1970 ..--4 ~1972
I*
”
.19?2
HERRING Fig. 9. Growth
,977
YEAR-CLASS ‘,
.)
,.
SPRATS
in length of O-group herring (autumn- and spring-spawned)
and sprats in the different
localities.
obse~ations of Marshall, Nichols & Orr (1939) and Ford (1928) for autumn-spawned herring of the Clyde and the Plymouth area, respectively. The observed mean vague of increase in Iength of 2.82 mm/week of spring-spawned herring is significantly lower than that of Clyde spring-spawned herring (3.12 mm/week). It was suggested by Marshall, Nichols & Orr that their samples were selective, and this could possibly have resulted in an apparent higher growth rate: also a study of the growth curves of Marshalf, Nichols & Orr shows that, except for three samples, all the rest consisted of unmetamorphosed fish; a curve based on such data, may result in erroneous conclusions.
1NSHORE
-
AUTUMN
CLUPEID
-SPAWNED
F.OF LORNE
POPULATIONS
-----SPRING
131
- .SPAWNED
L.ETlVE
30!
3oi
iJ
10
3 401
‘iL g
s
10
1 N
,,.A
,.,’
401
A 0
f 11 30
/ I 70 I 50
/ I 110 / I '30 IS 90
v--rrr-+ 150 I,170 60 80
LENGTH Fig. 10. Length-frequency
distribution
of O-group
;:‘\ 193 120
'i IT 140' ZOO'
(mm> autumn-
and spring-spawned
herring.
132
S. S. DE SILVA
L.SPELVE
L.ETIVE
1970 YR CLASS 1971
1
.,
A
0
,.’
S
,i‘\
.'L_
.' 80
”
.\\
-/-
10 30 t 20 40 60 m-r-r-r-
I
”
/
: \
,’ !?,
I
5-\ \
100 120
Fig. 11. Length-frequency
,u
!,I 14040 60
:\ ,' ..'_ ,FI III, 80 10 120 l40 , ,
LENGTH
distribution
(mm>20
p\ '_ , 40
, 50
, 80
, 100 120
/ 140
of O-group sprats of the 1970-71 year-classes.
INSHORE
CLUPEID
POPULATIONS
133
Growth of O-group sprats Unlike O-group herring, growth curves representing growth in length of O-group sprats of the 1970 and 1971 year-classes were different from one another (Fig. 9). This difference is a result of the change in spawning time in the two years (De Silva, 1973). Since sprats were caught in the area in 1970 only after August the number of samples obtained during the growth season, though far from satisfactory, still allows a smooth growth curve to be constructed, bringing the prominent characteristics to light. It is seen that growth in length ceased by late August-September except in Loch Spelve in 1970, where growth continued until November; this is thought to be exceptional and it is concluded that the growth check in O-group sprats occurs in late August-September. Though there is an early cessation in growth for the winter, resumption of growth takes place much earlier thanin herring, by late February-March. The rate of growth was calculated from the slope of the line during the main growing season for the 1970 and 1971 year-classes and was found to be 3.58, 3.50 and 3.80 mm/week and 3.50, 3.53 and 3.46 mm/week in Lochs Spelve, Etive, and the Firth of Lorne respectively. The difference in the rate of growth of any one year-class was not statistically significant from locality to locality, nor when one year-class was compared with the rest. It is evident that the rate of increase in length in O-group sprats in the area is independent of the spawning time and the average increase is 3.55 mm/week. A delay in spawning would, however, affect the mean length at which the first winter ring is laid down and, unlike herring, the lengths of sprats may differ appreciably from one year-class to another depending on the time of spawning. The length-frequency distribution of each 0.5 cm length group for the two yearclasses was determined (Fig. 11) for Lochs Etive, Spelve, and the Firth of Lorne. From these curves, the homogeneity of the samples and the increase in length up to AugustSeptember and also after the winter check are evident. On resumption of growth after the winter there is a rapid increase in length for a short period and then growth is slowed down considerably and maintained at a reduced rate. This increase was observed for both year-classes in all three localities. The increase is not consistent in its magnitude from year to year nor from locality to locality. It cannot be caused entirely by a migration of bigger fish into any one area, since a corresponding decrease in length was not observed elsewhere. This is considered to be a growth characteristic peculiar to sprat, the significance of which will be dealt with in the discussion. Growth of older clupeids The number of l-year-old and older herring caught in the area after April was so small that sufficient numbers were not obtained to study the growth rates after their first year of life and was further complicated because the herring had to be separated into their respective races; such separations based on meristic characters of small samples are far from reliable. As a result, this and following sections are concerned only with growth in sprats.
S. S. DE SILVA
I 14
Using empirical data (length) at the end of winters 1970-71 and 1971-72 length-age curves were drawn for females and males {Fig. 12). It is evident from Fig. I2 that the curves for the two sexes for any one growth season is similar. A third curve for each growth season was compiled by combining the sexes (Fig. 12). The combined curve for the two years are different, especially in their lower range. Such differences are to be expected since the spawning period in different years may not be the same and also year-class variation would be more noticeable in a short-lived species. The greatest absolute increase in length takes place in the first year and thereafter the increase tends to be progressively smaller from year to year. En general, the combined length-age curve at the end of the 1970-71 winter was very smooth, exhibiting characters of a .__.
1970 _ ‘71 WINTER
ee
19’71_ “.’. ‘N!N:: i:
MALES
7
0,
,
,
,
,
I
I-+
;4 .
~ SEXES
COMBINED
I
-80 EE -60 % 540 5 -220
AGE Fig.
12. Age-length
curves
IN YEARS
of females, males and the sexes combined, increases in each year of life.
with absolute
and percentage
INSHORE
CLUPEID
POPULATIONS
135
conventional growth curve, but at the end of 1971-72 winter showed signs of an inflection between the 4th and 5th year of life. Length-weight
relationship
The length-weight relationships of O-group sprats and herring were calculated separately from that of older fish. The weight of all the fish in each mm group throughout the year were averaged and plotted (Fig. 13). The length-weight relationships are given in Table I. The general equation describing the relationship is W = aL” where W = weight, a = intercept, L = length and n = an exponent (slope). The values for O-group herring of both races are similar to those given by Marshall, Nichols & Orr (1939). There are no data available for herring between one- and threeyear-old. Fraser (1931) however, found W = 0.00000337 L3,‘49 for adult herring from the English Channel. Johnson (1970) estimated the length-weight relationship as W = 0.0000029 L3.42 for Wash sprats during their non-feeding season. The difference in ‘a’ between the two populations is probably due to seasonal differences in the
12_
HERRING - AUTUMN ----
SPRING
8-
g
20
60
100
1 105
140
SPRATS
16-
22-
8-
35
105
175
1 245l_ENG;\
80
120
(mm) Fig. 13. Length-weight
relationship
of herring
and sprats.
1 160
s. s.
136
analysis
but other local factors
DE SILVA
such as temperature
and nature
of feeding
grounds
may also affect it. It is possible that the difference in both the ‘a’ and ‘11’values of autumn and spring-spawned O-group herring are at least partly genetic. TA5LF
The length-weight
relationship
of herring
I
and sprats
at different n
of the number
+_ r---e :j II
,
DORSAL FIN HAY5 r”A
SCALESi;*-!i
F fJF LORQ&. --
13
14
RAYS
-II_.... c..
of vertebrae,
dorsal and anal tin-rays,
and keeled scales (K2)
of
sprats from different localities (on either side of the mean the broad bar represents 2W and the thin line the range, where 2W = (p, n,)s; .q; p. number in the sampte: nZ. error degrees of freedom; s,, pooled
standard
error of the sample; q, upper percentage point of the Studentized I *;, significance level 1.
range
at the
INSHORE CLUPEID POPULATIONS
The only observation
so far on the meristic
characters
141
of any one sprat popula-
tion in British waters is that of Bowers (1949). He determined the mean number of vertebrae in O-group sprats in Manx Bay and found it to be 47.67. On the other hand, 1950) and Swedish (Molander, 1940; Lindquist, 1968) workers Danish (Poulsen, carried out populations sprat which keeled scale
extensive investigations on the meristic characters of different sprat from the Scandinavian fjords. Poulsen (1950) recognized five stocks of he thought to be racially different on the basis of mean vertebral and count differences. DISCUSSION
It has been shown that young herring begin to migrate into the area in April and sprats about July-August. Analysis of the herring stocks show that the herring which come into the area in April probably originate from the Minch autumn-spawning stock and that those from a spring-spawning stock arrive in June. The mean length when they first appear is between 40-45 mm with a range of 30-57 mm. The similarity in the mean length and the time of arrival of the broods in 1970,197l and 1972 cannot be rejected as a mere coincidence. Baxter (1954) observed the presence of autumnspawned larvae off the south west of Mull in the months of September and October, and Wood (1971) observed concentrations of autumn-spawned larvae in the same area in October. The virtual absence of larvae inshore in the Oban area (results of laboratory larval surveys 1969-72) when their movement is more liable to be determined by tidal drift etc., seems to indicate more than a passive drift of young herring into the area. The arrival of young herring is thought, therefore, to be at least partly due to an active migratory movement. Less is known about the sprats, though they seem to spawn outside the area of investigation (De Silva, 1973) and broods from different years move in at different lengths. The relative estimates of abundance of O-group clupeids give clear indications of the importance of this and similar areas as nursery grounds for young clupeids. The Moray Firth has been known to be one of the major nursery areas for young herring in Scottish waters and it has been shown recently that young herring in this area originate from the Minch autumn-spawning stock (Saville, 1971). When abundance estimates of O-group herring in the Moray Firth are considered it is seen that, except for two catches, less than 500 fish/h fishing were obtained and in October and December the catches were even less. Catches in April-May and afterwards as l-group fish, were much higher in the Moray Firth than in the Oban area. It is evident, therefore, that at least during the first year of life, Minch autumn-spawned herring are found in larger numbers in the Oban area than in the Moray Firth (assuming the gear is not more efficient). Dragesund (1970) has shown that abundance estimates of O-group herring in the Norwegian fjords give vital clues to the year-class strength of any one brood long before recruitment to the adult stock. In British waters prediction of stock abundance
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has been mainly based on larval abundance studies. Such estimates are subject to much error because of the extensive mortality at this phase of life. It is suggested, therefore, that long-term research on abundance of O-group fish in inshore areas similar to those around Oban and correlation of such data with those after recruitment may give more reliable estimates of stock abundance and year-class strength. O-group herring move into the area investigated and then a localization in their distribution takes place within a very short time. On the other hand O-group sprats tend to remain in appreciable numbers in all the sea-lochs and ‘open’ areas. A comparison of growth rates of herring and sprats in their first year of life has shown that the rates are not significantly different from locality to locality. The rate growth is primarily determined by the food supply and the physical conditions of the environment. The environmental conditions in the sea-lochs and ‘open’ areas under consideration are not greatly different from one another (Gage, 1972). It is possible that the similarity in the growth rates of O-group herring and sprats in the different localities indicates that a stable food supply is available which is not limiting their growth. There is evidence that the zooplankton biomass per unit volume in Loch Etiveis higher than in Loch Creran (Mauchline, 1972). It has also been shown that O-group herring and sprats depend on the same food organisms (De Silva, 1973) and that when and if the supply is limiting, competition between them is a possibility. if, therefore, O-group herring and sprats were to occur at the same intensity in all the localities, food is likely to be a limiting factor for growth and even survival. so that it is possible that the movement of young herring from the smaller lochs, i.e., Spelve, Creran. ctc’. may be a consequence of the availability of a limited food supply common to both species and thereby a means of avoiding direct competition. There may be other factors operating, e.g., herring prefer deeper waters as they grow older and may tend to leave the small lochs. The tendency of young herring after being established in the larger lochs and ‘open’ areas to emigrate during the winter months has been discussed. The absence of deep waters (> 70 m) in most of the areas sampled and the concentration of herring to a certain extent in the deep trench in Loch Etive during the winter, provide further evidence in favour of the tendency of young herring to move into deeper water. The greater availability of food in Loch Etive compared with the other areas during the winter months (Mauchline, 1972) may also act as a contributory factor. The tendency to move into deeper waters is probably less pronounced in O-group sprats. in sprat, maturation takes place in the second year of life (De Silva, 1973) and the movement to the spawning grounds may also be a contributory factor to the movement from inshore waters. It is probable that herring in particular will have an open-sea phase 01’ life before being recruited to the adult stock, while in sprats recruitment takes place directly and earlier in life. The similarity of sprat populations to commercial catches has been shown (Fig. 8) but it must be pointed out that the similarity in year-class distribution for two seasons is far from sufficient for any definite conclusions to be made. If this were found to be
of
1NSHORE CLUPEID
POPULATIONS
143
by research true in subsequent long-term studies, it is suggested that sampling vessels in inshore waters, similar to the ones presently investigated, should give good indications of year-class strength of sprats and, therefore, predictions of the yield for the fishery well in advance. The mean length of the 1970 and 1971 year-classes at the end of the first winter was 64.0 and 70.0 mm. It was shown earlier that a rapid increase in length in O-group sprats is known to occur after their first winter. This rapid increase is probably due to a very high increase in feeding activity (De Silva, 1973) soon after the winter during the planktonic outburst of the spring. The sprat is a short-lived species and tends to mature in its second year of life (1 + ) at a minimum size of 90 mm (De Silva, 1973). This rapid increase in length after the winter probably enables most of the 1+ individuals to reach maturity in their second year of life. Herring and abundant gadoids such as cod, haddock, and whiting show large fluctuations in year-class strength but the effects of such large fluctuations on the total adult stock is usually smoothed out by the presence of several year-classes in any given population. Poor survival in certain years tends to be compensated for by good survival in others. The sprat, with a few year-classes, is more sensitive to a year of poor recruitment, so that populations of such species as the sprat need to ensure a rapid replacement of the stock i.e., fast growth and early maturity. On the other hand, the longer living herring mature later in life and recruitment each year represents a smaller proportion of the population in general. ACKNOWLEDGEMENTS I would like to express my sincere thanks to the following: to Dr J. H. S. Blaxter for his advice, encouragement and for critically reading the manuscript; to Dr H. Barnes for his help in the preparation of the manuscript and to Dr J. D. M. Gordon for his invaluable assistance in the field. Financial assistance provided by the University of Ceylon is gratefully acknowledged. REFERENCES BARNES,H. & E. F. W. GOODLEY,1961. The general hydrography of the Clyde Sea Area, Scotland. Part I. Description of the area, drift bottle and surface salinity data. Bull. Mar. Ecol., Vol. 5, pp. 112-150. BAXTER,I. G., 1954. Herring larvae at the Scottish West Coast., Ann. biol. Copenh., Vol. 10, pp. 150-151. BAXTER,I. G., 1958. The composition of the Minch herring stocks. Rapp. P.-v. RPun. Cons. perm. inf Explor. Mer, Vol. 143, pp. 81-94. BAXTER,I. G., 1963. Features of the minch herring fisheries in pre- and post-war years. Rupp. P.-v. R&n. Cons. perm. int. Explor. Mer, Vol. 154, pp. 227-235. Bowers, A. B., 1949. Notes on the migration of O-group sprats into the Manx Bays. Ann. Rep., Mar. Biol. Stn, Port Erin, No. 62, pp. 18-19. BOWERS,A. B., 1952. Studies on the herring (Clupea harengus L.) in Manx Waters: the autumn spawning and the larval and post-larval stages. Proc. Trans. L’pool biol. Sot., Vol. 58, pp. 41-74. DANIEL, R. J., 1947. Growth rate of whitebait herring of the north Wales coast. J. Cons. perm. int. Explor. Mer, Vol. 15, pp. 42-60. DE SILVA, S. S., 1973. Clupeid populations of inshore waters of the west coast of Scotland. Ph. D. Thesis, University of Stirling, 122 pp.
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DRAC~ESLJND,O., 1970. Recruitment studies of Norwegian spring spawning herring (Clupeu harengus Linnt?). Fisk. Dir. Skr. Ser. HavUnders., Vol. 15, pp. 381-554. FORD, E., 1928. Herring investigations at Plymouth. IV. The growth of young herrings in the neighbourhood of Plymouth. 1. mar. hiol. Ass. U.K., Vol. 15, pp. 305-309. FRASER, J. H., 1931. On the size of Urosalpinx cinerea (Say) with some observations on length-weight relationship. Proc. Malacol. SIX., Vol. 19, pp. 243-254. GAGE, .I.,1972. A preliminary survey of the benthic macrofauna and sediments in Lochs Etive and Creran, sea-lochs along the west coast of Scotland. J. mc~r. hiol. Ass. U.K., Vol. 52, pp. 237-276. HARDING, J. P., 1949. The use of probability paper for graphical analysis of polymodal frequency distributions. J. mar. biol. Ass. U.K., Vol. 28, pp. 141-153. HEINTKE, F., 1898. Naturgeschichte des Herings. Ahh. dtsrh. See&h. Ver., Bd 2, S. 128-227. JOHNSON, P. O., 1970. The Wash sprat fishery. Fishery Invest., Load., Ser. -7, Vol. 26, 88 pp. LANGHAM, N. E., 1969. The west coast sprat fishery. Scar. Fisher.v Bull., Vol. 32, pp. 19-21. LINDQUIST, A., 1968. Meristic and morphometric characters, year-classes and ‘races’ of the sprat (Sprattus sprattus). Inst. mar. Res. Lysekil, Ser. Biol., Rep. 17, 25 pp. MARSHALL, S. M., A. G. NICHOLS &A. P. ORR, 1939. On the growth and feeding of young herring in the Clyde. J. mar. biol. Ass. U. K., Vol. 23, pp. 427-455. MAIJ~HLINE, J., 1972. Assessing similarity between samples of plankton. J. twr. hiol. .4rs. Inditr, Vol. 14, pp. 26-41. MOLANDER, A. R., 1940. A research upon the sprat of the west coast of Sweden. I. SwnshtrH.l;dr.-Bioi. Komm. skr. N. S., Biologi, Vol. 2, 63 pp. PARRISH, B. B. & SAVILLE, A., 1965. The biology of the northeast Atlantic herring populations. In, Ocecrnogr. Mar. Biol. Arm. Rep., edited by H. Barnes, George Allen and Clnwin Ltd., London. pp. 323.-373. PARRISH, B. B., A. SAVILLE, R. E. CRAIG, I. G. BAXTEK & R. PRIESTLEY, iY59. Observations on herring spawning and larval distribution in the Firth of Clyde in 1958. J. ,~NT. biol. Ass. U. K., Vol. 38, pp. 445-453. PARRISH, B. 8. & D. P. SHARMAN, 1958. Some remarks on methods used in hcrrmg racial investigations, with special reference to otolith studies. Rapp. P.-r. Rktn. Cons. perm. in/. E.xp/or. Mer, Vol. 143, pp. 66-80. PARRISH, B. B. & D. P. SHARMAN, 1960. Otolith types amongst summer-autumn spawning herring in the North Sea. J. Cons. perm. int. Explor. Mer, Vol. 25, pp. 81-92. PEARSON, T. H., 1970. The benthic ecology of Loch Linnhe and Loch Eil. a sea-loch system on the west coast of Scotland I. The physical environment and distribution of the macrobenthic fauna. J. ‘xp. mar. Biol. Ecol., Vol. 5, pp. I-35. POULSEN, E. M., 1950. The sprat fishery and the sprat populations In the Danish waters. Rep. Dunish Biol. Stn, Vol. 52, pp. 5-24. RISKER, W. E., 1958. Handbook of computation for biological statistics of tish populations. J. Fish. Res. Bd. Canad. Bull. No. 119, 300 pp. ROBERTSON, J. A., 1938. The sprat and sprat fishery of England. Fishery fm~est., L.owd., Ser. 1. Vol. 16, 103pp. SAVILLE, A., 1971. The biology of young herring in the Moray Firth and their recruitment to the adult stocks. Rapp. P.-v. RPun. Cons. perm. int. Explor. Mer, Vol. 160, pp. 184.. 193. Sv~~ov~oov, A. N., 1952. Fauna qf U.S.S.R. Fishes. Vol. II, No. 1. Clupeidue. Israel Programme for Scientific Translations, 1963, 375 pp. SYMONDS, D. J., 1964. Racial studies on Manx herring stocks. J. C‘ons. perm. int. tluplor. Mer. Vol. 29, pp. 189-204. WHEIXER. A., 1969. The fishes of the British Isles and north-west Europe. Macmillan, London. 613 pp. WOOD, H., 1960. The herring of the Clyde estuary. Mar. Res. Scut., No. 5, 24 pp. WOOD, R. J., 1959. Investigations on O-group herring. J. Cons. perm. int. Exp/or. Adrr. Vol. 24, pp. 2644276. WOOD, R. J., 1971. Whitebait herring on the English east coast. Rapp. P.-c. Rirm. Cons. perm. ifrt. Erplor. Mer, Vol. 160, pp. 166-l 70. WOOD, R. J., 1972. Herring larvae to the west of Scotland in the autumn of 197 I. IC’ES,Pelagic Fish (Northern) Commt., 1972/H:6, II pp.
