References

Study of the growth in length of dab Limanda limanda (L.) off the north-east coast of Anglesey, North Wales

© Florence Folmer, December 2000

Abstract

The growth in length of female dab Limanda limanda (L.) off the north-east coast of Anglesey, North Wales was investigated. A manual method similar to the Ford-Walford method and a computerised method (non-linear regression method using SPSS software) were used to estimate the parameters of the von Bertalanffy equation. Both methods were compared. The results obtained for female dab were compared with the results for male dab, as well as with results obtained in the same survey area over the last six years, and in other survey areas. The results were also analysed in relation with various biological factors observed in the survey area off the north-east coast of Anglesey, North Wales. The purpose of the study was to analyse the current growth in size of female dab Limanda limanda (L.) off the north-east coast of Anglesey, North Wales, as part of a long-term fish population dynamics survey in the mentioned area. The estimated parameters of the von Bertalanffy equation for female dab Limanda limanda (L.) off the north-east coast of Anglesey, North Wales, in October 2000 are as follow:

Lmax = 31.53 cm; K = 0.40 years-1 ; t0 = - 0.23 years (manual method)

Lmax = 31.03 cm; K = 0.40 years-1 ; t0 = - 0.28 years (computerised method)

Introduction

Because fish stocks in European waters are becoming increasingly exploited and many of them are showing clear signs of over-fishing (Macer, 1967; Rijnsdrop et al., 1992), there is a great interest in the factors which determine their productivity. Most of the mortality probably takes place in the larval and juvenile stages, and thus it might be practicable to rear young fish obtained from hatcheries or from nursery areas, with the objective of reducing the natural mortality and, possibly, increasing growth rate. It is important, therefore, to have a full understanding of the ecology of the fish (Macer, 1967). Fish differ from such terrestrial animals as mammals and birds in that they appear to continue growing throughout the whole of their lives. Age data, in conjunction with length and weight measurements, can give information on stock composition, age at maturity, life span, mortality, and production. Analysis of the growth rate of a fish population may result in a better interpretation and prediction of ecological consequences of growth for the population (Bakhsh, 1982).

Red Wharf Bay, on the coast of Anglesey, Wales, and Conwy Bay, on the coast of North-Wales, are nursery grounds which support populations of young fish - principally plaice (Pleuronectes platessa) and dab (Limanda limanda) - resulting from spawning in the off-shore waters off Great Orme Head (about fifteen kilometre to the north-east of Red Wharf Bay) (Macer, 1967; Bolle et al., 1994).

The growth rate of dab Limanda limanda (L.) has been studied by several authors, including Bohl (in the North Sea (Dogger Bank), in 1957), Kandler and Thurow (in the western Baltic (Kiel Bay), in 1959), Jonsson (in Icelandic waters, in 1966), Lee (in central and southern North Sea, in 1972), and Ortega-Salas (at the Isle of Man, in 1980) (Bakhsh, 1982). The growth of the dab Limanda limanda (L.) population in Anglesey waters has been studied by Bakhsh (1982).

According to Bolle et al. (1994), female dab reach sexual maturity at age two to three, while males reach maturity by age two. Dab are high-fecundity serial spawners with a prolonged spawning period. Egg production takes place from January to September, with major spawning activity in February to April (Bakhsh, 1982; Bolle et al., 1994). Spawning occurs offshore. Postlarvae settle at a size of 13-20 mm, shortly before metamorphosis is completed. Settlement of dab is known to occur in shallow open bays along the coasts of Scotland and Wales.

According to Bakhsh (1982), one of the acceptable methods to record the age of dab Limanda limanda in the Anlesey water dab Limanda limanda population is to count the number of opaque bands on the otoliths. The central opaque nucleus is followed by alternating hyaline hyaline winter rings, which appear as white bands under reflected light against a dark background, and opaque summer rings, which appear as dark bands.

In the present report, the survey of the growth rate of female dab Limanda limanda L. off the north-east coast of Anglesey, Wales, is presented. The age of the dab has been recorded by otolith reading. The parameters of the von Bertalanffy equation have been estimated by a manual method similar to the Ford-Walford method, and by a computerised non-linear regression method, using SPSS software. The results obtained by both methods have been compared with each other, as well as with results from previous surveys.

Material and methods

The cruises took place on October 3rd to October 18th 2000. The sampling stations are shown in the appendix ("task 1"). Cruise 5 took place on October 12th 2000 at the Molfre/ Port lynas station, on the research vessel Prince Madog of the University of Bangor, Wales. On October 12th 2000, the weather and sea conditions were moderate. During the two weeks preceding the day of the cruise, and on the day itself, the weather was rainy, windy, and cold. The research vessel Prince Madog left the pier of Menai Bridge at 9 am on October 12th 2000, and it took about one and a half hours for it to reach the sampling station of Molfre/ Port lynas. Fishing took place from approximately 10:30 am to 11:30 am, between the positions 53°26'N 03°60'W and 53°28'N 04°03'W, at a depth of 36-44 m, using a Rockhapper (demersal) otter trawl gear. Ab otter trawl consists of a conical or funnel-shaped net leading into the cod-end in which the fish are retained. Demersal otter trawls are designed for bottom trawling, in which the target species include a large variety of demersal fish species such as flatfish and cod, as well as invertebrates (King, 1995). Prince Madog sails by automatic steering, except in the Menai Straits, where it is steered manually due to the unpredictable conditions prevailing in the Straits. Measurements are taken by GPS, electronic and magnetic compasses, and a small weather station. A computer program provides information about the navigation route, the rain fall, the location of nearby vessels,...

The catch was sorted by species, measured (total length to the nearest cm; the data was reported on provided length/frequency forms), and either stored in separate buckets or bags (only three fishes per size class were retained for each fish species), or released, after qualitative analysis, in the case of elasmobranchs or other by-catch. Elasmobranchs, and in particular rays and skates, are, indeed, extremely vulnerable to over-fishing (in addition to species-targeted fishing, the anatomical structure of rays and skates causes them to be easily caught in nets-even when fishing is not targeted on them), but they are capable of surviving properly proceeded catch & release.

After landing, at approximately 1:30 pm, the catch was transported to the marine biology laboratory of the UWB School of Ocean Sciences. In the laboratory, the retained fishes were measured (total length to the nearest 0.1 cm, depth to the nearest 0.1 cm), weighted (to the nearest 0.1 g), and sexed. The stomach content was evaluated in percentage of fullness. The identified content was ranked on a scale adding up to 10. The maturity stage (I-III) of each fish was evaluated using comparison charts and pictures. The otoliths were collected and stored in the envelope containing the rest of the data described above.

In a second laboratory stage, the age of the fishes was determined by counting the year rings on the otoliths. To do so, the otoliths were placed into a histoclear solution and observed under a binocular microscope. A schematic representation of a dab otolith is presented in figure 1 (Figure 1 is not available in the online version of the present paper.).

N.B.: By definition, the birthday of dab off the north-east coast of Anglesey has been determined, for the present study, to be on April 1st of each respective year. Hence, all the fishes caught in the October cruise were aged n++ (n.5 - n.6 years; n being the number of completed years of life).

The study of the dynamics of dab Limanda limanda (L.) populations off the north-east coast of Anglesey was then separated into different tasks.

In order to study the growth in length for the dab populations (male/female) sampled during the cruises (task 4), the parameters (Lmax, K, t0) of the von Bertalanffy equation (equation 1) have to be estimated, using the total length (L) and the age (t) (based on the otolith rings) data collected previously in the lab.

equation 1:

where Lt is the total length (in cm) of the fish at the age t (in years)

Lmax is the theoretical maximal total length (in cm) the fish can reach

K is a growth coefficient which measures the rate at which Lmax is reached

t is the age (in years) of the fish

t0 is the theoretical age at length zero

(Equation 1 shows that growth in length gradually slows down as fish get larger.)

The parameters of the von Bertalanffy equation can be estimated using two different methods. One method of estimating the parameters of the von Bertalanffy equation for data representing equal time intervals is by means of two different plots (or by means of one Ford-Walford plot) (the "classical" methods). The derivation of the first plot is based on the von Bertalanffy equation with t0 equal to zero (equation 2):

equation 2:

where Lt is the total length (in cm) of the fish at the age t (in years)

Lmax is the theoretical maximal total length (in cm) the fish can reach

K is a growth coefficient which measures the rate at which Lmax is reached

t is the age (in years) of the fish

After substituting Lt+1 for Lt in equation 2, the difference between this new equation and equation 2 is given by equation 3:

equation 3:

where L t+1 is the total length (in cm) of the fish at the age t+1 (in years)

Lt is the total length (in cm) of the fish at the age t (in years)

Lmax is the theoretical maximal total length (in cm) the fish can reach

K is a growth coefficient which measures the rate at which Lmax is reached

t is the age (in years) of the fish

Substituting equation 2 into equation 3 gives:

equation 4:

( y = b + a ∙ x)

where L t+1 is the total length (in cm) of the fish at the age t+1 (in years)

Lt is the total length (in cm) of the fish at the age t (in years)

Lmax is the theoretical maximal total length (in cm) the fish can reach

K is a growth coefficient which measures the rate at which Lmax is reached

Equation 4 is of a linear form, and suggests that the difference between the length at age t+1 (Lt+1) and at age t (Lt) can be plotted against the length at age t (Lt). The straight line fitting these data will have a slope of , and the intercept of the line on the x-axis provides an estimated of Lmax:

Intercept on the x-axis =

 

Equation 2 can also be expressed as:

equation 5:

( y = b + a∙ x)

where Lt is the total length (in cm) of the fish at the age t (in years)

Lmax is the theoretical maximal total length (in cm) the fish can reach

K is a growth coefficient which measures the rate at which Lmax is reached

t is the age (in years) of the fish

t0 is the theoretical age at length zero

Equation 5 is of a linear form, and suggests that the natural logarithm of the difference between the maximal length and the length at age t (Lt) can be plotted against the age t. The straight line fitting these data will have a slope of a = - K, and t0 can be obtained from equation 6:

equation 6:

where Lmax is the theoretical maximal total length (in cm) the fish can reach

K is a growth coefficient which measures the rate at which Lmax is reached

t0 is the theoretical age at length zero

b is the intercept on the y-axis of the straight line fitting the data

The second method used to estimate the parameters of the von Bertalanffy equation for data representing equal time intervals is to fit a curved regression directly to the original data set, using iteration fitting techniques, such as SPSS for Windows or PFit. The von Bertalanffy equation used in the SPSS non-linear regression statistics programme to generate a value for Lmax, K, and t0 is:

Y=Lmax*(1-Exp(-K*(X-t0)))

The initial values for the iteration are determined as follows:

- Lmax is estimated from a scatter plot of the original data.

- K is also estimated from the scatter plot of the original data, by the

equation , where x is the age at which the fish reach half of Lmax.

 

Results

a) The catch obtained by bottom trawl off the north-east coast of Anglesey, Wales

The catch of the cruise on October 12th 2000 included Red gurnard (Aspitrigla cuculus) (very abundant), Grey gurnard (Eutrigla gurnardus), Tub gurnard (Trigla lucerna) (very abundant), Dab (Limanda limanda) (very abundant), Plaice (Pleuronectes platessa) (very abundant), Cod (Gadus morhua), Whiting (Merlangius merlangus), Red mullet (Mullus surmuletus), Dragonet (Callionymus lyra), Angler fish (Lophius piscatorius), Bull rout (Myoxocephalus scorpius), Haddock (Melanogrammus aeglefinus), Sole (Solea solea), Solenette (Buglossidium luteum), Lesser spotted dogfish (Scyliorhinus caniculus), Large spotted dogfish (Scyliorhinus stellaris), Cuckoo ray (Raja naevus), Thornback ray (Raja clavata), worms (Pectinaria koreni, Nephtys sp., very abundant), squids, starfish, common scallop (Aequipecten opercularis), and anemones (Metridium sp.).

b) Growth in length of female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales - data collected in October 2000

The mean size of female dab (Limanda limanda L.) caught off the north-east coast of Anglesey, Wales, in October 2000, is shown in table 1.

Table 1. The mean size of female dab (Limanda limanda L.) caught off the north-east coast of Anglesey, Wales, in October 2000.

Age (years)

N

Mean size (Lt) ± S.E. (cm)

L(t+1)-Lt (cm)

0.53

10

8.60 ± 0.47

7.20

1.53

47

15.80 ± 0.53

5.13

2.53

87

20.93 ± 0.33

3.32

3.53

47

24.25 ± 0.42

2.67

4.53

15

26.92 ± 0.59

1.16

5.53

4

28.08 ± 1.09

-

Table 1 shows the number (N) of fish per age-class measured, the mean total length (Lt) of the fish from each age-class (± S.E.), and the difference between the total length for the next age-class and the total length for the concerned age-class (L(t+1)-Lt).The results are given at a 95% confidence level.

 

b1) The parameters of the von Bertalanffy equation obtained using the "classical method"

The plot of (L(t+1)-Lt) against (Lt) for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales is shown in figure 2.

Figure 2. Plot of (L(t+1)-Lt ) against (Lt) for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales - data collected in October 2000.

The regression equation for the plot of (L(t+1)-Lt ) against (Lt) is: y =10.0-0.319 x

According to equation 4,

Lmax = Intercept on the x-axis = -(b/a) = -(10.0/-0.319) cm

= 31.53 cm

The plot of ln(Lmax-Lt) against age for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales is shown in figure 3.

Figure 3. Plot of ln (Lmax-Lt ) against age for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales - data collected in October 2000.

The regression equation for the plot of ln (Lmax-Lt ) against age is: y =3.36-0.398 x

According to equation 5,

K = - a

= 0.40 years-1

and according to equation 6,

= (3.36 - ln (31.53))/0.40 years

= -0.23 years

The von Bertalanffy equation parameters for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales, estimated using the "classical method", are as follow:

Lmax = 31.53 cm

K = 0.40 years-1

t0 = - 0.23 years

 

The growth curve for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales, using the parameters estimated using the "classical method" is shown in figure 4.

b2) The parameters of the von Bertalanffy equation obtained using the "classical method"

The von Bertalanffy equation parameters for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales, estimated by iteration using the SPSS non-linear regression program are shown in table 2.

Table 2. The von Bertalanffy equation parameters for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales (data collected in October 2000), estimated by iteration using the SPSS non-linear regression program.

Parameter

 

Estimate Asymptotic ± S.E. (95% C.I.)

Lmax (cm)

 

31.03 ± 2.08

K (years-1)

 

0.40 ± 0.07

t0 (years)

 

- 0.28 ± 0.18

Asymptotic correlation matrix of the parameters estimated

 

t0

K

Lmax

-0.74

-0.96

K

0.88

 

t0

   

Source of variation

Deg. F

Sum of squares

Mean square

Treatments

3

91681.9

30560.6

Residuals

208

1946.8

9.4

Total

210

5802.7

 

R2 = 0.664

     

The growth curve for female dab (Limanda limanda L.) off the north-east coast of Anglesey, Wales, using the parameters estimated by iteration using the SPSS non-linear regression program is shown in figure 4.

 

Figure 4 shows the mean total length for each age class of female dab, and the von Bertalanffy growth curve using the manual "LogFit" method and the computerised non-linear regression method using SPSS software.

c) Growth in length of Limanda limanda L. off the north-east coast of Anglesey, Wales (based on the PFit/SPSS non-linear regression method) - data collected between 1994 and 2000

The von Bertalanffy equation parameters for Limanda limanda L. off the north-east coast of Anglesey, Wales, based on data collected between 1994 and 2000, are shown in table 3.

Table 3. von Bertalanffy equation parameters for Limanda limanda L. off the north-east coast of Anglesey, Wales, estimated by using the PFit/SPSS non-linear regression method) - data collected between 1994 and 2000.

Female Dab (Limanda limanda L.)

(von Bertalanffy equation: )

Year

 

Lmax (cm)

K (years-1)

T0 (years)

1994

 

29.69

0.32

-0.91

1995

 

24.53

0.10

-1.48

1996

 

24.60

1.04

0.33

1997

 

28.90

0.50

-0.42

1998

 

26.20

0.55

-0.11

1999

 

29.79

0.47

-0.02

2000

 

31.03

0.40

-0.28

Male Dab (Limanda limanda L.)

(von Bertalanffy equation: )

Year

 

Lmax (cm)

K (years-1)

T0 (years)

1994

 

22.61

0.43

-1.36

1995

 

22.79

0.71

-0.29

1996

 

23.53

0.71

-0.17

1997

 

23.00

0.73

-0.30

1998

 

22.00

0.62

-0.49

1999

 

19.21

1.03

-0.10

2000

 

27.30

0.45

-0.37

source: unpublished U.W.B. student surveys

The von Bertalanffy equation parameters for Limanda limanda L. estimated previously by other authors are shown in table 4.

Table 4. von Bertalanffy equation parameters for Limanda limanda L. estimated previously by other authors.

Bakhsh (1982, Anglesey waters):

Lmax = 28 cm; K = 0.30 years-1 ; t0 = - 0.88 years (female dab)

Lmax = 21 cm; K = 0.42 years-1 ; t0 = - 1.25 years (male dab)

 

Rijnsdrop et al. (1992, inshore south eastern North Sea):

Lmax = 30.5 cm; K = 0.28 years-1 ; t0 = - 0.07 years (female dab)

Lmax = 25.0 cm; K = 0.26 years-1 ; t0 = - 0.34 years (male dab)

 

Discussion

The results obtained for the catch, the by-catch, and the gut content (see task 6 in appendix) during the survey correspond to the description of the general ecology of Anglesey waters by Macer (1967). As mentioned by Macer (1967), the benthos in the survey area north-east of Anglesey is of "boreal offshore muddy sand" type.

The present study (task 5 and task 2) confirms the results obtained by Bakhsh (1982) regarding the maturation rate and the growth pattern of dab Limanda limanda in Anglesey waters. Males mature at an earlier age (two years) and lower length (11 cm) than females (two-three years, 13 cm). The body depth and head grow isometrically with total body length in both sexes. In the present survey, the majority of dab were 2++ years old. Only few 4++ and 5++ years old dab were caught, and the oldest dab were 5++ years old. Although this age distribution might be related to the sampling depth, to the sampling site, and to migration factors, it is worth noticing that the oldest caught dabs were 5++ old, which is rather young in relation with the maturity age of dab, and in relation with dab from other populations. Surveys by Rijnsdrop et al. (1992) in the south-eastern North Sea, for example, included up to 10 years old dabs, with a majority of 4-5 years old dabs.

For both female and male dabs sampled during the present survey, the von Bertalanffy parameters estimated by the manual method and the computerised method were very similar, meaning that the two methods are equivalent in terms of studying the growth rate of dab Limanda limanda (L.). The use of both methods together allows a double-proof. The computerised method tends to be faster than the manual method, which, itself has the advantage that it can be performed without a computer, and without special non-linear regression software. The manual method does, however, not always work. In particular, when the sample-size is small, or when only a few age classes are collected, the manual method is difficult to be used. Amongst the manual methods available, several methods similar to the one used in the present survey can be used. The Ford-Walford method, for example, can also be used, and it relies on a single graph, instead of the two graphs needed in the method used in the present study. According to King (1995), the Lmax and t0 values obtained by non-linear regression methods of fitting growth curves is that there are usually very few small and large fish in the samples obtained for analysis. As a consequence, the Lmax and t0 values often represent large extrapolations beyond the range of the sample data.

The age of the dabs in the present survey was recorded by counting the opaque otolith bands. According to Bakhsh (1982), this method is acceptable to record the age of dab Limanda limanda in the Anglesey water dab Limanda limanda population is to count the number of opaque bands on the otoliths. As stated by Francis (1988), in order to use the number of otolith bands to evaluate the growth rate of a fish, it is very important to have the certitude that the otolith rings actually correspond to year rings, as in the case of Anglesey water dab Limanda limanda (L.) populations, in order to avoid falsified results. To verify that the otolith reading method is accurate for the evaluation of a fish's age, one can compare the number of otolith ring with the length of the fish for randomly chosen small/medium/large fish. Methods similar to the otolith reading method include the study of scales (King, 1995). Other methods available to study the growth rate of fish include tagging. Results obtained by tagging and by otolith reading cannot be directly compared with each other, though (Francis, 1988).

Both female and male dabs sampled during the present survey appear to be to have a slightly higher infinity-length (Lmax) than dabs sampled in the same survey area in previous years (1982 (Bakhsh (1982)), 1994-1999 (unpublished surveys by U.W.B. students), as well as in other survey areas (Rijnsdrop et al. (1992), inshore south eastern North Sea). Nevertheless, it is not possible to conclude significant differences between the results of the present survey and previous results. Indeed, as mentioned by Bakhsh (1982), variations in growth rate between different populations have been reported by several authors for various fish species, including dab Limanda limanda, cod Gadhus morhua, and plaice Pleuronectes platessa. In particular, Bakhsh (1982) has noticed variations in data from U.W.B. student surveys of the growth rate of dab in Anglesey in the 1970's. As explained by Bakhsh (1982), the variation may well be due to the sampling time, sampling depth, sampling size, or even to inexperience in measurement skills, such as otoliths reading. If further studies were to show a significant increase in growth rate of dab, this increase could be related to changes in food conditions, or to changes in exploitation of dab or of other flatfish in the survey area. Indeed, according to Bakhsh (1982), less crowded fish stocks and less competition for food between dab and other flatfish can account for increase in growth rate. Advantages of a fast growth rate may include, for example, reaching a size early in life which gives the species some immunity from its predators; generally, larger individuals suffer less predation than smaller ones. Attaining a large body size may allow a large number of eggs to be carried, or the production of larger eggs, with correspondingly higher chances of larval survival. From a fisheries point of view, growth, as well as recruitment, influences the sustainable catch weight that can be taken from the stock (King, 1995).

The present survey shows a significantly higher growth rate for female dab than for male dab. This agrees with the results obtained in previous surveys by U.W.B. students (1994-1999, Anglesey waters), as well as by several authors, including Bakhsh (1982, Anglesey waters), Bohl (1957, in the North Sea (Dogger Bank)), Kandler and Thurow (1959, in the western Baltic (Kiel Bay)), Jonsson (1966, in Icelandic waters), Lee (1972, in central and southern North Sea), and Ortega-Salas (1980, at the Isle of Man) (Bakhsh, 1982). Bakhsh (1982) has also stated that female dab have a higher length at maturity than male dab.

In the present survey, as well as in all the previous surveys mentioned above, the value for the parameter t0 for both female and male dab is negative, meaning that juveniles grow faster than predicted for adults.

As a conclusion of the present survey, one can say that it has provided an insight into the population dynamics of the Anglesey waters dab Limanda limanda (L.) population. Nevertheless, in order to thoroughly understand the population dynamics of this dab population, one needs to analyse, in depth, all the aspects of population dynamics (i.e. mortality rate, maturation rate, diet,...) in relationship with each other.

 

References

Bakhsh, A.A. 1982. Population studies of the Limanda limanda (L.) in Anglesey waters. Ph.D. Thesis University of Bangor, Wales.

Bolle, L.J. et al. 1994. Nursery grounds of dab (Limanda limanda L.) in the southern North Sea. Netherlands Journal of Sea Research, 32, 299-307.

Francis, R.I.C.C. 1988. Are growth parameters estimated from tagging and age-length data comparable? Canadian Journal of Fisheries and Aquacultural Sciences, 45, 936-942.

King, M. 1995. Fisheries biology, assessment and management. Oxford: Fishing News Books. pp. 79-197.

Macer, C.T. 1967. The food web in Red Wharf Bay (North Wales) with particular reference to young plaice (Pleuronectes platessa). Helgoländer Meeresuntersuchungen, 15, 560-573.

Rijnsdrop, A.D. et al. 1992. Population biology of dab Limanda limanda in the south-eastern North Sea. Marine Ecology Progress Series, 91, 19-35.

 

Appendix

 

Results for tasks 1-6 for dab Limanda limanda (L.) off the north-east coast of Anglesey, North Wales, using the data collected in October 2000

Task 1: Sampling records

Task 2: Fish condition; Growth in weight and net selection factor

Task 3: Death rate

Task 4: Growth in length

Task 5: Percentage maturity curve according to age

Task 6: Food preference and diet