Estimating biomass and production of pelagic larvae of brachyuran decapods in western European shelf waters

J. Exp. Mar. Biol. Ecol., 1988, Vol. 122, pp. 195-211 Elsevier

195

JEM 01146

Estimating biomass and production of pelagic larvae of brachyuran decapods in western European shelf waters J. Alistair Lindley Natural Environment Research Council, Plymouth Marine Laboratory, Plymouth, U.K. (Received

18 February

1988; revision

received

13 June 1988; accepted

25 July 1988)

Abstract: Methods are described for estimating the dry weight biomass, production due to growth, production due to moults, flux to the plankton from the benthos of hatching zoeae and from the plankton to the benthos of settling megalopae of common Brachyura from north-western European waters. These methods are applied to brachyuran larvae taken in Continuous Plankton Records in the Irish Sea during 1981-83 and from the Longhurst Hardy Plankton Recorder over Great Sole Bank in June 1983. The former provided estimates of input of hatching zoeae to the plankton at 10 m depth from the benthos of 0.41 mg dry weight,m-3.a-’ with a peak in April, a mean standing stock of 0.19 mg. m- 3 with a maximum in May. The peak in production was in July and the annual ratio of production to biomass (P : B) was 23.2 : 1 excluding moults, or 26.1 : 1 including moults. The estimated flux of settling megalopae from the plankton at 1Omdepth tothebenthoswas 1.33mg.m-3.a-’ with a peak in July. The data from the LHPR haul gave instantaneous daily values of an input of 0.004 mg m- a. day - ’ to the plankton and a mean biomass 6.696 mg m -’ of brachyuran larvae in the plankton. The daily P: B ratio was 0.067 : 1 excluding moults and 0.073 : 1 including moults and an estimated 0.07 mg m *. day ’ was returning to the benthos. Key words: Benthic-pelagic

interaction;

Biomass;

Brachyura;

Larva;

Production

Warwick et al. (1986) presented evidence for benthic/pelagic coupling in the Celtic Sea and speculated that interactions between the planktotrophic larvae of the macrobenthos and the holoplankton might be a contributory mechanism. Radford et al. (in press) demonstrated that inclusion of estimates of flux between the benthos and the plankton corrected previously unrealistic model output of many state variables in simulation of the ecology of the Bristol Channel and Severn Estuary. The meroplankton can constitute a substantial proportion of the zooplankton biomass, particularly in inshore waters. Williams & Collins (1986) found that the meroplankton, the dominant constituents of which were decapod larvae, accounted for 58 and 23 y0 of the “omnivore biomass” in two regions of the Bristol Channel (U.K.) and Tenore et al. (1982) found that the larvae of the anomuran Pisidia longicomis (L.) constituted 90% of the zooplankton biomass in the Ria de Arosa (N.W. Spain). Correspondence address: J. A. Lindley, Natural Environment Research Laboratory, Prospect Place, West Hoe, Plymouth PLl 3DH, U.K. 0022-0981/88/$03.50

0 1988 Elsevier

Science

Publishers

B.V. (Biomedical

Council,

Division)

Plymouth

Marine

196

J.A.LINDLEY

Estimates of biomass and production are most precise when relationships between length or development stage and weight of each species and their development rates can be determined under ambient conditions using the samples of the extant population. The diversity of species of Brachyura and the problems of identification of larvae of closely related species (e.g. Liocarcinus spp., Clark, 1984) make it unlikely that comprehensive data could be determined simultaneously. The objective of this paper is to describe methods for making preliminary estimates of the biomass and production of pelagic larvae of Brachyura, including flux between the plankton and the benthos from plankton survey data where the spatial and temporal resolution are inadequate to estimate development times and mortality directly. Data are presented here on the relationships between carapace lengths and dry weights of brachyuran zoeae. Regressions calculated from these data and published data are used to provide a method of estimating dry weights of megalopae. Estimation of dry weight biomass from field data is described using dry weight values derived by these methods. Published data on development rates and mortality are used to estimate production in the plankton. Rates of recruitment of first zoeae to the plankton and of return to the benthos at the end of larval development and the flux of biomass involved are estimated. Estimates of the biomass and production of pelagic larvae of brachyura made by these methods could potentially contribute towards quantifying the role of the meroplankton. In the tabulated data from Bodo et al. (1965) and the original counts which provided the data for Rees (1952) and Lindley (1987) 9 taxa (genera or species) accounted for over 90% of brachyuran larvae in surveys in north-western European waters. Other surveys in which brachyuran larvae were identified but for which insufficient data were published to make quantitative statements (e.g., Thorson, 1946; Rees, 1955; Williamson, 1956; Baan et al, 1972) indicate that the same taxa were predominant. Of the 9 taxa 4, Liocarcinus spp. (previously recorded as Portunus spp. or Macropipus spp.), Cancer pagurus L., Atelecyclus spp. (mainly A. rotundatus (Olivi)) and Corystes cassivelaunus (Pennant) have 5 zoeal instars and a megalopa; 4 taxa, Curcinus maenus L., Pirimela denticulata (Montagu), Xantho spp. and Pilumnus hirteZZus(L.)have 4 zoeae and a megalopa and Hyas spp. (mainly H. coarctatus Leach) has 2 zoeae and a megalopa. The pre-zoea is assumed to be a transitional phase in hatching and not a distinct instar. The estimates of biomass and production for the common taxa are used as a basis for estimating values for total brachyuran larvae.

MATERIALS

AND

METHODS

The relationship between carapace lengths and dry weights was determined using brachyuran zoeae sorted live from hand-net samples. These samples were taken off Plymouth on 26 April 1982 and 7 May 1982 and on a research cruise on R.R.S. ‘Challenger’ in the Great Sole Bank area of the Celtic Sea, 6-20 June 1983. Each specimen was identified and the length from the anterior to posterior margin of the

ESTIMATING

BIOMASS

AND PRODUCTION

OF PELAGIC

LARVAE

197

carapace was measured to the nearest 0.1 mm. Sorting, identification and measurement were all performed using a Wild M5 stereomicroscope with an eyepiece graticule. The specimens were dried and weighed as described by Williams & Robins (1982). Regressions describing the relationships between logarithms of carapace lengths and dry weights of zoeae were calculated by least-squares regression. Only 2 megalopae were measured in the present survey so data from the other sources were used to provide an estimate of the relationship between the mean dry weight of the last zoeal instar and that of the megalopa. The duration of each developmental stage was calculated using published regressions relating duration to temperature. The regressions described by Sastry (1976) for Cancer irroratus Say were applied for taxa with 5 zoeal instars, those described by Dawirs (1985) for Curcinus menus for taxa with 4 zoeal instars and those described by Anger (1984a) for Hyas coarctutus were applied to that species. Equations used in the estimation of production are listed in Table I. In order to calculate growth rate an estimate was made of the mean age from hatching or moult of each instar. Recruitment from the previous developmental stage was assumed to be constant, so taking account of mortality during intermoult growth, the mean age would be less than the mid-point of the development time of the instar. Survival on successive days was calculated by equation (1) and the mean age was calculated by equation (2). Few data on natural mortality rates are available so a single value, the mean

TABLE I Equations n, =

used to estimate

parameters

n, , e -.z

x, = {Z$[(C r, = (z;-

(1) + l).n,]/Z~n,}

‘D,) + xi

P, = Zr(6

- 1

(2) (3)

W, ai/Di)

(4)

n,

(5)

Pi, = n&~ P, = Zy

of production.

’ ai. Pi,. 0.1 . Wi

H = ai n,&~

n,

(6) (7)

B, = W,,.H

(8)

S=a;P,,

(9)

B, = W,;S

(10)

n, = number surviving on day t; Z = instantaneous daily mortality rate; xi = mean age from moult of instar i, CO= day of hatching or moulting to instar i; Di = duration of development of instar i; Ti = mean age from hatching of instar i; Pg = daily production attributable to somatic growth; M = megalopa, 6 W, = increment in dry weight during instar i; ai = abundance of instar i; Pi, = daily proportion of instar m moulting; P, = daily production attributable to moults; H = daily number of ZIs hatching; E, = biomass of daily hatch of ZIs; W,, = predicted dry weight of ZI on hatching; S = daily number of megalopae moulting to Arst young crab stage; B, = biomass of megalopae moulting to first young crab stage; W,, = predicted dry weight of megalopa at moult to young crab stage.

198

J. A. LINDLEY

instantaneous daily mortality rate derived by Nichols et al. (1982) for Cancer paguru,s larvae in the North Sea (0.028) was used throughout in the calculations in this paper. The growth rate for each taxon was calculated by fitting an equation of the form In DW = a + bt, where DW = dry weight (mg), t = age (days from hatching) and a and b are constants fitted using the mean dry weight of an instar plotted at the appropriate value oft calculated by equation (3). This was used to calculate the dry weight at each moult. Total production attributable to somatic growth (P,) was then calculated from equation (4) which is comparable to that developed by Petrovich et al. (1964) to calculate the production of copepods. Data given by Dawirs (1983) show that the dry weights of discarded exoskeletons of zoeae of Carcinus maenas were approximately 4-6x of the late pre-moult dry weights of the moulting instars whereas Anger (1984b) showed that zoeae of Hyas aruneus (L.) lost about 14% of late pre-moult dry weight. An intermediate value of 10% was used to calculate P, in equation (5) which is based on that used by Sameoto (1976) to calculate the production of moults of euphausiids. Equation (5) is modified from that used by Sameoto by including adjustment for mortality during development, which results in a lower value for the numbers moulting daily. P, values were calculated for the zoeae only as the megalopae were assumed to moult on return to the benthos. The number of zoeae hatching daily and recruited to the plankton Q was calculated by equation (7) and the predicted dry weights of first zoeae at hatching (W,) were used to calculate the biomass that this represented (equation 8). Settlement from the plankton and return to the benthos were assumed to take place at the moult of the megalopa to the first young crab stage; the number reaching this stage was calculated by equation (9) and the dry weight biomass involved was calculated by equation (10). Examples of plankton survey data to which these methods were applied to illustrate the method were taken from the Continuous Plankton Recorder (CPR) survey (Glover, 1967) and samples taken with the Longhurst-Hardy Plankton Recorder (LHPR) (Williams et al., 1983). The CPRs are towed by ships-of-opportunity at 10 m depth on standard routes at monthly intervals. Samples containing the plankton retained by gauze of 280 pm aperture from 3 m3 of water are taken for each 16 km of tow. Samples containing decapod crustacea taken during 1981-1983 have been re-examined and the decapods were identified to species where practicable but to genus or sub-family in some cases. Developmental stages were noted and the carapace lengths, where these were not seriously distorted, were measured to the nearest 0.1 mm. The distributions and seasonal cycles of decapod taxa taken in this survey have been described by Lindley (1987). The data used in this paper to illustrate the application of the methods were those for brachyura taken in the Irish Sea (Area C3, see Fig. 1). In each month the mean dry weight of each zoeal instar of each of the common taxa (those noted above but excluding Pirimelu which was rare in these samples) was calculated by application of the regression relating weight to carapace length to the carapace length/frequency data for each developmental stage. Development times were calculated using the mean sea surface

ESTIMATING

BIOMASS

AND PRODUCTION

OF PELAGIC

LARVAE

199

60.

55. lN --

50. ..N--

45. #‘N----L IS.‘U Fig. 1. Standard areas C3 and D4 of the Continuous Plankton Recorder Survey. The position of the LHPR haul used in this paper is shown by the black dot on the boundary of Area D4.

temperature value for the area for that month. This value was derived from temperature data supplied by the U.K. Meteorological Oflice. An equation describing the growth rates for each taxon was calculated for each month using the development times and dry weights for each instar. Where insufficient data for a month were available, dry weight values were interpolated using data from preceding months for instars earlier than the modal stage in the month and data from succeeding months for later instars. The mean numbers per sample and stage frequency data were used to calculate the mean biomass (mg . m- 3), daily Pg, P,, B, and B, values (mg * m- 3 . day - ‘) for each taxon. Monthly and annual mean biomass and monthly and annual production values for each taxon and for total Brachyura were calculated incorporating an estimate for rarer Brachyura assuming that the values for the mean biomass and production parameters per larva for the rarer taxa were equal to the mean values for the common taxa. The LHPR is towed on an oblique haul taking discrete samples at regular intervals from which the vertical distributions of planktonic organisms can be described. Decapods from a LHPR haul at 49” 15’N; 11 ‘W (see Fig. 1983 have been identified to specific or generic level and vertical distributions have been previously described by weight values for each developmental stage of each of the were calculated from length/frequency data from the CPR

1) at 08 14 GMT on 9 June development stage. Their Lindley (1986). Mean dry common brachyuran taxa survey for June in area D4

J. A. LINDLEY

200

(Fig. 1) in 1981-1983. The dry weight for each 5-m depth interval and the total values (mg*mp2) were calculated. Development times at each depth interval were derived using temperature data from the thermograph within the recorder control unit. From these values, estimates of daily production for each 5-m depth interval and a total value for production under 1 m2 including values of B, and B, were calculated. Values for the rarer larvae were calculated in the same way as they were for the CPR data.

RESULTS

The dry weights and carapace lengths of brachyuran zoeae are plotted on logarithmic equation fitted to these data was scales in Fig. 2. The regression log,,DW = - 0.963 + 2.591ogieCL where DW = dry weight and CL = carapace length. Dry weights of megalopae given by other authors were approximately double the corresponding weights of the last zoeal instars (Table II). The mean dry weight of megalopae was assumed to be double the estimated mean dry weight of the corresponding last zoea throughout in this paper. The estimates of biomass, Pp, P,, B, and B, calculated from CPR data for the Irish Sea and the contribution to each of the dominant taxa are presented in Fig. 3. The monthly values for the production parameters are the products of the estimated daily

O-l 1

x

111 1”

.I

-2.0 -013

-a!2

d

-O!l

Log. Carapace

O!l

0!2

Length

Fig. 2. Data points, regression line and 95% confidence limits for the relationship between carapace length (mm) and dry weight (mg) of brachyuran zoeae. The taxa, sampling locations and dates of the data can be identified from Table II.

ESTIMATING

BIOMASS AND PRODU~ION

201

OF PELAGIC LARVAE

values and the number of days in the month. These results show a ratio of annual production to mean biomass (P: z) of 23.2 : 1 excluding moults or 26.1:1 including moults for total brachyuran larvae in the Irish Sea.

0

Others Liocarcinus

E1.o In

Cancer

x

LJ m

Atelecyclus

‘E Corystes

E

Hyas

(FO.i! j-

TOTAL 0.51 mg

E n x ... 70

rri3K’

0

4 TO PLANKTON

‘E AT

10m FROM BENTHOS

FROM PLANKTON

F

AT

1Om TO BENTHOS

o JFMAMJJASOND

I

JFMAMJ

J

ASOND

JFMAMJJASONO

Fig. 3. Brachyuran larvae. Seasonal variations and annual totals of biomass and production at 10 m depth in the Irish Sea (Area C3) from CPR data 1981-1983. Dm = duration of month m (days).

J. A. LINDLEY

202

TABLE II Brachyuran

decapods.

Dry weights (mg) of last zoeal instars (Z) and megalopae and calculated M/Z values.

Species Rhithropanopaeus harrisii’ Macromedaeus distinguendus’ Hemigrapsus longitarsis’ Sesarmops intermedius’ Carcinus maena? Carcinus maenaP Carcinus maenaP Carcinus maena? Hyas araneu? Hyas coarctatus4 Hyas coarctatus3 Menippe mercenaria’ Cancer irroratu9 Cancer irroratus’ ’ Mean 3 Early s Late ’ Mean

Z

M

0.075 0.131 0.108 0.046 0.044 0.073 0.088 0.052 0.292 0.204 0.093 0.104 0.333 0.386

0.148 0.285 0.171 0.095 0.066 0.145 0.145 0.123 0.528 0.392 0.214 0.208 0.443 0.464

(M) from published

Reference

WZ 1.79 2.18 1.56 2.07 1.52 1.99 1.64 2.37 1.80 1.92 2.31 2.0 1.33 1.22

sources

Levine Hirota Hirota Hirota Dawirs Dawirs Dawirs Dawirs Anger Jacobi Anger Mootz Sastry Sastry

& Sulkin (1979) & Fukuda (1985) & Fukuda (1985) & Fukuda (1985) et al. (1986) (9 “C) er al. (1986) (12°C) et al. (1986) (18 “C) (1983) (1984b) & Anger (1984) (1984a) & Epifanio (1974) (1979) (1979)

value from graph. * Mean from plankton samples. post-moult. 4 Mean of tabulated values. pre-moult. 6 Mean at constant temperature. with die1 cycles.

The dominant brachyuran larvae taken in the LHPR haul were Liocarcinus spp. and Atelecyclus rotundatus which accounted for 76.8 and 16.9x, respectively, of the total numbers in the haul. The remainder were Monodueus couchi (Couch) and Goneplux rhomboides (L.). For each 5-m depth interval the biomass under 1 m2 was calculated and development times for Liocarcinus spp. and Atelecyclus were derived and used to calculate Pg, P,, B, and B,. The only first-stage zoeae present were those of Liocarcinus spp. so the B, value is for that genus only. The B, value for Atelecyclus was 87% of the total with Liocarcinus contributing the remainder. The mean values per larva of biomass, Pg and P, for the rarer species were assumed to be the same as for the common taxa at the same depth. The results are shown in Fig. 4.

DISCUSSION

The accuracy of the estimates of biomass and production derived by the methods used here are dependent on the regression relating carapace length to dry weight providing a reasonably accurate dry weight value; on the predictions of development times being appropriate for the taxa to which they are applied and on growth between moults being approximately linear. The first requirement was tested by comparing other published dry weight values which could be related to corresponding carapace lengths. The results, excluding data for starved larvae, (e.g. Dawirs, 1983; Anger, 1986) are shown in Fig. 5. The carapace

ESTIMATING

BIOMASS AND PRODUCTION

BIOMASS

TO PLANKTON

8,=0.004

mg

mg

rii%’ 0.8

mg

FROM

m4’

0

EENTHOS

203

pe

F9 Iii*

0

OF PELAGIC LARVAE

0.05

mg

FROM

0.008

PLANKTON

B,=O.O7mg

rlc2,_~

0

TO BENTHOS

6%’

Fig. 4. Brachyuran larvae. Vertical distribution and total values of biomass an daily production rates from a haul at 49” 15’N;ll “OO’W, 9.6.83,0814 GMT.

length of an individual remains constant during intermoult growth which usually results in approximately doubling the post-moult dry weight but greater increases have been noted (e.g. Incze et al., 1984). Consequently dry weights derived throughout intermoult growth should be a range spanning the regression. In many cases this occurred but there were examples of consistent deviation, for example the weight ranges of successive zoeal instars of Carcinus maenas fell increasingly below the values predicted by the regression. Such consistent deviations may be attributed tentatively to morphological variation such as the absence of lateral carapace spines on Carcinus zoeae in contrast with their presence on the majority of specimens from which the regression was calculated. Despite the considerable variations in weight during intermoult growth, len~h/weight regressions or mean weights for each instar are commonly used in estimating the biomass and production of planktonic crustacea (e.g. Burkill & Kendall, 1982; Mauchline, 1985; Roff ertal., 1988). Development times of some north-western European species are given in Table III. The values for the common species for which data are presented approximate fairly well to the results from the regressions used in this paper. However, the development times for Inachus spp., Macropodia rostrata and Pisa armata (Hartnoll & Mohamedeen, 1987; Ingle, 1982; Ingle & Clark, 1980, respectively) show that these developed more quickly than Hyas spp. at the same temperature. As a generalisation, however, the regressions give reasonable estimates of development times, given the variability under constant conditions found between broods of the same species (e.g. Dawirs, 1985), effects of temperature change and its direction (e.g. Anger, 1983), salinity (e.g. Johns, 1982) and other environmental variables.

-~-~-.~.-

Species

Temp. (“C)

Development times of north-western

TABLE III

1.1 9.1

Nichols et al. (1982)’

(1987)

6.9

8.8

Sastry (1976)’

13.1

51.6

3.7

Hartnoll

8.6 8.8-9.6

(1985)2

2.8

3. I

Dawirs ( 1985)2

2.5

3.0 3.1

4.1 3.6 2.6

3.0

3.1

4.0-4.6 4.4

Hartnoll % Mohamedeen Dawirs (1985)’

3.6

4.2

3.5-4.9

4.4

6.6 4.4

6.0

7.2

8.7

8.8-10.1

8.1

5.1

8.5

5.6

4.1

4.1-6.1 4.8

4.4

6.2 4.2

6.5 5.5

7.6

8.1-9.0

7.6

5.2

4.6

4.5

Dawirs (1985)’

Dawirs (1985)* Dawirs ei al. (1986)

Dawirs (1983)

(1987)

6.2 5.9 4.8

Dawirs (1985)’

4.4

7.2

Hartnoll & Mohamedeen Dawirs ( 198.5)1

8.6

Dawirs et al. (1986)

Dawirs ( 1985)2

Dawirs

Dawirs (1985)’

(1987)

4.9

& Mohamedeen

4.8

Nichols el ul. (1982)

Ingle & Rice (1971)

3.4

3.0

5.1

4.5

5.4

5.1-8.9

5.3

5.4

9.9

1.5

9.6

11.2

11.3-13.0

11.2

5.9

6.7 6.0

5.8

7.4

14.3

10.1

it.8

12.1-13.8

12.9

11.8

15.3

17.6

21.2

19.5-27.6

21.2

_

18.2

18.5

30.1

26.2

30.6

28.8-38.3

31.4

30,6

40.7

48.1

56.5-69.3 57.3

57.3

38.4

6.0

(1976)’

sastry

5.2

20.6

104.3

c

39 5.3

8.9

8.0

42.1

-_

Rice & Ingle (1978) 4.5

6.5

9.2

14.5

-

M

50

5.9

8.5

9.7 6.7

zv

48

(1975b) 4.5

9.6 6.8

i3.9

9.7

ZIV

lnstar

Rice & In&

6.1

11,2

15.3

11.7

2111

I-^

Rice & Ingle (1975b)

Ingle & Rice (1971)

6.1

11.1

Ingle & Rice (197 I)

13.7

14.8 14.8

11.5

211

Sastry (1976)’

21

Nichols et al. (1982)’

Reference

European brachyuran larvae in relation to the values predicted by the regressions of Sastry (1976), Dawirs (1985) and Anger (1984).

r z tr

3

z

Christiansen

Anger (1984)*

Anger (1983)

6

9

9

10

10

10

12

12

12

15

15

15

15

15

15

15

15

18

18

18

Hyas amneus

Hyas coarctatw

Hyas coarctatus

Hyas coarctatus

Hyas coarctatus Hyas araneus

Hyas coarctatus

Hyas coarctatus

Hyas amneu~

Hyas coarctatw Hyas comctatus

Hyas araneus

Inachus dorsettensis

Inachw dorsettensis

Inachus leptocheirus

Pisa armata

Macropodia rostrata

Hyas coarctatus

Hyas coarctam Hyas waneus

’ Calculated * Tabulated

(1973)

(1973)

(1973)

& Mohamedeen

& Mohamedeen

& Mohamedeen

Christiansen

20 20 (1973)

where regressions

are given.

Hart11011 & Mohamedeen

Anger (1984)’

Ingle (1982) Anger (1984)’

Ingle & Clark (1980)

Hartnoll

Hartnoll

Hartnoll

Christiansen

Anger (1984)’ Christiansen (1973)

Anger (1984)*

Anger (1984)*

Anger (1984)’

Anger (1984)’

Anger (1984)’

Anger (1984)’

Anger (1983)

Anger (1984)’

20

from regressions. values from papers

Inachus dorsettensis

Hyas coarctatus Hyas coarctatu~

Christiansen

6

Hyas coarctatus

Anger (1984)’

6

Anger (1983)

2

Hyas amneus

Hyas coarctatus

(c) Species with 2 zoeal instars

(1987)

(1987)

(1987)

(1987)

2.0

12.0

7.0

7-9

8.4-9.3

8.1

6.0

5.7

4.6

4.7

15.3

13.6

10.0

11-15

12.4-14.9

13.1

20.8

19.7

16.3

19.0-20.6

18.5

26-32

30.4-35.7

30.1

61-70

3.3

6.9 7.5

8-12

7.9 8-X.1

4.0

7.2

5.5

5.2

12.5

11.3

10.2

13-17

13.5-14.2

13.8

17.0

18.6

17.8

21.0-21.6

20.5

30-39

35.5-36.5

35.7

77-89 42.8

5.0

12.0

(18-21)

13.4

13.4

11.0

10.3

9.2

29.2

21.1

16.2

22-35

20.5 20.4-26.3

(27.2)

37.4

24.9

28.0

27.8

39-50

42.8-55.3

108.6

10.3

25.9

34-47

29.4 29.8-30.x

22.30

21.0

20.4

19.1

57.0

46.0

36.4

51-60

(65) 47.4 46.3-55.4

75.7

59.0

68.0-70.2

66.8

98-111

108.7-127.5

206

J. A. LINDLEY

Log. Carapace Length Fig. 5. Brachyuran zoeae. Relationships between carapace lengths and dry weights, comparison of present regression (continuous line) with regression from Hirota & Fukuda (1985) (broken line) and other published values (vertical bars): (1) Carcinus maenas (dry weights from Dawns, 1983; Dawirs et al., 1986; carapace length from Rice & Ingle, 1975a), (2) Hyas spp. (dry weights from Anger, 1984; Kunisch & Anger, 1984; Jacobi & Anger, 1985; carapace lengths from Christiansen, 1973), (3) Cancer irrorurus (dry weights from Sastry, 1979; Johns, 1982; carapace Iengths from Sastry, 1977), (4)~en~pe ~erce~ar~u (dry weights from Mootz & Epifanio, 1974; carapace lengths from Porter, 1960; (5) Chiuaoecetes spp. (dry weights from Incze & Paul, 1983; Incze etal., 1984; carapace lengths from Haynes, 1973; Incze et al., 1984).

Few data on the magnitude and causes of natural mortality of crustacean meroplankton are available. In this paper the value of 2 (0.028) is value calculated by Nichols et al. (1982) from ZI to ZIV of Cancerpagums in their survey of the western North Sea. Their data indicated highest values at earliest stages and lower values as development continued. Nichols et al. (1987) give a mean 2 value of 0.035 for Nephrqs nomegicus (L.) zoeae in the Irish Sea. The data of Rothlisberg & Miller (1983) show higher values of 0.046 and 0.071 from surveys of larval Pandalu~jordani Rathbun in 2 yr while the mean inferred from 20 yr of fisheries data was 0.055. It is probable that mortality is highest at ecdysis when stress and vulnerability are higher than between mot&s. Further data on mortality under natual conditions are needed to enhance the reliability of estimates of production parameters. The differences in results between incorporating the constant value of 2 to adjust the parameters for mortality during development and simply dividing the numbers of an instar by its development time were small for zoeae at summer temperatures in the Irish Sea. The differences were greater for zoeae in the spring when temperatures were lower and for megalopae.

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In the calculation of Pg (Table I, equation 4) growth of an instar is taken as linear. This may seem paradoxical when an exponential rate of growth has been calculated and used to estimate the initial and final weights of the instar. Exponential equations accurately describe overall growth as measured by single weight parameters such as mean dry weight (Dawirs, 1983) or maximum achieved carbon content (e.g. Dawirs etal., 1986) for each instar. However, Dawirs et al. (1986) and Jacobi & Anger (1985) showed that intermoult growth in conditions of excess food supply is most rapid immediately after a moult and subsequently slows or even reverses in contrast with exponential growth the rate of which increases with time. The values of the coefftcients of the quadratic equations fitted to intermoult growth vary with temperature (Dawirs et al., 1986) but the few currently available data do not provide the basis for making generalisations applicable to survey data. The linear model for intermoult growth used here gives production estimates intermediate between higher values produced by the laboratory determined data and lower values derived by assuming exponential growth. The dry weights at moult, predicted by fitting an exponential equation to the data of Dawirs et al. (1986) for Curcinus at 12 “C assuming that the mean of the tabulated dry weight values for each instar was attained at the mid-point of the development time of that instar, are compared with the actual values for late pre-moult and early post-moult dry weights in Table IV. This shows that use here of exponential equations to predict the weights of newly hatched zoeae and settling megalopae overestimate the weights in each case. The use of the same value for the final weight of one instar and the initial weight of the next instar omits any adjustment for the loss of weight of the discarded exoskeleton. Estimates of production of crustacean zooplankton generally assume continuous growth (e.g. Ritz & Hosie, 1982; Franz & Diel, 1984) but as further detailed studies of crustacean growth become available the discontinuities at ecdysis may be incorporated into more precise calculations. The transition from pelagic to benthic is taken to occur at the moult of the megalopa but some species may remain pelagic until later stages (e.g. Rice & Hartnoll, 1983). The results of application of the methods described here to CPR data must be qualified by the limitations of the method. Single depth sampling results in the relationship between the numbers per sample and the total population varying as a result of interspecific, ontogenetic, die1 and seasonal variations in vertical distributions. No account is taken of variability of temperature with depth and the effects on development TABLE IV Carcinus maenas. Comparison of dry weights (pg) at moulting predicted by exponential equation pre-moult and early post-moult values from the data of Dawirs et al. (1986). Moult Predicted Late pre-moult Early post-moult

E-Z1

ZI-ZII

ZII-ZIII

14.7

22.8 19.1 19.0

33.5 29.9 34.5

10.6

ZIII-ZIV 52.2 54.5 53.4

with late

ZIV-M

M-Cl

92.2 74.4 84.5

270.7 138.8 182.9

208

J.A.LINDLEY

rates of vertical migrations in thermally stratified waters. Not enough is known of the variability of vertical distributions to take account of them in calculations but it is known that megalopae frequently tend to live at greater depths than zoeae (Russell, 1925; Nichols et al., 1982; Lindley, 1986; Harding & Nichols, 1987) and therefore a smaller proportion of the population of megalopae than of zoeae would be taken at 10 m depth. Therefore the estimates of the B, values may probably be smaller in proportion to the biomass and production at 10 m than values calculated from samples covering the whole depth range. At the other end of the cycle the B, values were dominated by Hyus coarctutus. This is partly because first stage zoeae of Hyas spp. are substantially larger, with carapace lengths of x 0.9-l mm than those of most of the other common taxa most of which have carapace length of ~0.5 mm. However, the numbers of first zoeae of most taxa were less abundant in the CPR samples than would be needed to give rise to the numbers of later stages. This may be a result of differing vertical distributions, localisation of hatching in areas not sampled by the CPR with subsequent dispersive migration to the CPR routes or lack of temporal resolution, peaks of first zoeae occurring between sampling occasions. The results of using the methods with CPR data are most likely to be useful in determining the timing of events, large scale temporal and spatial variability and estimates of P: B ratios. The production values for brachyuran larvae sampled by the LHPR haul taken over Great Sole Bank are expressed as rates per day. Lindley (1986) has shown that die1 variations in vertical distributions of brachyuran larvae occur. This means that populations in situations of complex vertical temperature structure will exhibit growth rates varying with time according to the temperatures prevailing at the depths that they inhabit at different times. The production values should therefore be regarded as instantaneous values for the time of sampling rather than integrated rates over a 24-h period. This problem will occur with any sampling in stratified waters. It is notable, however, that the daily P : B ratios calculated from these data multiplied by 365 are 24.5 : 1 and 26.6 : 1 which are very close to the annual values derived from the CPR data from the Irish Sea. The methods described in this paper will, therefore, give values which should be treated with appropriate reservations as estimates rather than precisely measured values. With these reservations the values derived may be used together with the data on chemical composition and energy budgets given in many of the papers cited here to provide preliminary estimate of, for example, the flux of nitrogen, carbon and energy through pelagic larvae of brachyura.

ACKNOWLEDGEMENTS

The CPR survey would not be possible without the willing cooperation of the owners, masters and crews of the vessels which tow the recorders. Many members of staff of PML are responsible for the maintenance of the survey which is partly financed by the U.K. Ministry of Agriculture, Fisheries and Food. I thank Mr. S.H. Coombs for the

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opportunity to participate in and use samples from the cruise on R.R.S. ‘Challenger’and Mr. D. Robins for passing on to me samples of live plankton taken off Plymouth. Much of the data processing depended on facilities provided and staffed by NERC Computer Services at PML. This work comprises part of the work of the Community Ecology group at PML. I thank colleagues and referees whose comments and criticisms of previous versions ofthe paper have led to corrections of errors and other improvements.

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