15
Netherlands Journal of Sea Research 9 (1) : 56-68 (1975) STUDIES ON THE BLACK GOBY (GOBIUS NIGER, GOBIIDAE, PISCES) IN THE VEERSE MEER, SW NETHERLANDS* by K. F. VAAS, A. G. VLASBLOM and P. DE KOEIJER (Delta Institute for Hydrobiological Research, Yerseke, The Netherlands) II. III. IV. V. VI. VII. VIII. IX. XI. CONTENTS Introduction . . . . . . . . . . . . . . . . . 56 The environment . . . . . . . . . . . . . . . 57 Distribution . . . . . . . . . . . . . . . . . 58 Morphological features of Gobius niger from the Veerse Meer 59 Food . . . . . . . . . . . . . . . . . . . 60 Reproduction . . . . . . . . . . . . . . . . 61 Growth and llfe span; relationship between weight and length. 61 Seasonal and yearly fluctuations in numbers . . . . . . . . 64 Changes in the demersal fish fauna of the lake after the establishment of Gobius niger . . . . . . . . . . . . . . . . 65 Summary . . . . . . . . . . . . . . . . . . 66 References . . . . . . . . . . . . . . . . . 67 I. INTRODUCTION In autumn 1964 a professional fisherman sent us a dark coloured gobiid fish, caught in a I~rke in the Veerse Meer. Although the specimen was damaged we tentatively identified it as Gobius niger L., the black goby, new to the Dutch fauna (VAAs, 1964). The bottom fish fauna of the lake was sampled with a shrimp trawl every month on 9 to 13 stations, but only in May 1966 3 more black gobies were caught and in December 1969 one more. However, since autumn 1970 the black goby never fails to appear in our catches, sometimes in numbers of more than 100 fish per 100 minutes trawling. Actually this new fish is at present more abundant in the Veerse Meer than the sand goby and the common goby (Pomatoschistus minutus and P. microps) previously the only representatives of the family in the lake. Since 1972 the black goby has been observed by divers in the canal connecting the Veerse Meer with the Westerschelde, and along the south coast of the Oosterschelde. * Communication no 126 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.

Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

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Page 1: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

Netherlands Journal of Sea Research 9 (1) : 56-68 (1975)

S T U D I E S O N T H E B L A C K G O B Y ( G O B I U S N I G E R ,

G O B I I D A E , P I S C E S ) I N T H E V E E R S E M E E R , S W

N E T H E R L A N D S *

by

K. F. VAAS, A. G. V L A S B L O M and P. DE K O E I J E R (Delta Institute for Hydrobiological Research, Yerseke, The Netherlands)

I° II.

III . IV. V.

VI. VII.

VIII . IX.

X° XI.

C O N T E N T S

Introduction . . . . . . . . . . . . . . . . . 56 The environment . . . . . . . . . . . . . . . 57 Distribution . . . . . . . . . . . . . . . . . 58 Morphological features of Gobius niger from the Veerse Meer 59 Food . . . . . . . . . . . . . . . . . . . 60 Reproduction . . . . . . . . . . . . . . . . 61 Growth and llfe span; relationship between weight and length. 61 Seasonal and yearly fluctuations in numbers . . . . . . . . 64 Changes in the demersal fish fauna of the lake after the establishment of Gobius niger . . . . . . . . . . . . . . . . 65 Summary . . . . . . . . . . . . . . . . . . 66 References . . . . . . . . . . . . . . . . . 67

I. I N T R O D U C T I O N

I n a u t u m n 1964 a professional f i sherman sent us a da rk coloured gobi id fish, c augh t in a I~rke in the Veerse Meer . A l though the spec imen was d a m a g e d we ten ta t ive ly identif ied it as Gobius niger L., the black goby, new to the D u t c h f auna (VAAs, 1964).

T h e b o t t o m fish f auna of the lake was s amp led wi th a sh r imp t rawl every m o n t h on 9 to 13 stations, bu t only in M a y 1966 3 more black gobies were c a u g h t a n d in D e c e m b e r 1969 one more . However , since a u t u m n 1970 the b lack goby never fails to a p p e a r in our catches, somet imes in n u m b e r s of more t h a n 100 fish per 100 minutes t rawling. Actua l ly this new fish is a t present more a b u n d a n t in the Veerse M e e r t h a n the sand goby and the c o m m o n goby (Pomatoschistus minutus and P. microps) previous ly the only representa t ives of the family in the lake. Since 1972 the b lack goby has been observed by divers in the cana l connec t ing the Veerse M e e r wi th the Westerschelde, and a long the south coast o f the Oosterschelde .

* Communication no 126 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.

Page 2: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

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,. -.

Page 3: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands
Page 4: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

G O B I U S N I G E R 57

This paper reports on the newly established population of Gobius niger in the Veerse Meer.

Acknowledgements . - -The authors are indebted to Dr M. Fonds ( T e x e l ) , whose many critical remarks greatly improved our manu- script. We also wish to thank C. de Rooy and J. A . van Sprundel, skipper and assistant on board the R.V. "Jan Verwey".

II . T H E E N V I R O N M E N T

The Veerse Meer is a former branch of the Oosterschelde with an open connection to the North Sea (for a map of the area see VAAs, 1970) and a salinity of approximately 307oo S. In 1961 the area was closed off by the construction of a dam with sluices through its con- nection with the Oosterschelde and a dam without sluices through its connection with the North Sea. The tidal rhythm disappeared and

29.

28.

27.

26.

2G.

24.

23.

22.

21.

20.

t 20.

18.

14.

12.

10.

8.

6.

/..

2.

0

i

'ill J ' F

, [ , I I

I

I ' M ' A ' M ' J ' J '

i I Ii

, ,I! I i '

II I' ,i,I I' ]! !

A J S ' O ' N ' D '

i I : I[ :

I ' 11

j ' F I M I A ' M ' j ' j ' A L S ' O ' ' N ' D '

Fig. 1. Maxima and minima of salinity and temperature measured on certain days in the upper I0 m layer of the Veerse Meer during 1972 and 1973 (broken lines),

after data from C. Bakker, Yerseke.

Page 5: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

5 8 K . F . VAAS, A. G. V L A S B L O M & P. DE K O E I J E R

salinity decreased to about 20 to 25 ~/0o S. The changes in the demersal fish fauna have been described by VAAS (1970). The Veerse Meet is now a brackish lake with a size of about 24 × 1.5 km and a mean depth of 4.5 metres, with some gullies of 6 to 20 metres depth. The annual fluctuations in temperature (2 to 20 ° C) and salinity (20 to 29~/~ S) are shown in Fig. 1. The water level in the lake is lowered in winter by 0.70 m and raised again in spring, partly with sea water from the Oosterschelde flowing in through the sluices, partly with drainage water from the surrounding polders. Eutrophication, due to drainage water and sewage from villages and campings, is fairly high; the phosphate levels vary from 5 to 20 ~zgat PO4-P/I. The transparency of the water generally varies from 2 to 6 m, but often decreases to 1 to 2 m during plankton blooms in spring. During these blooms the water is usually supersaturated with oxygen, whereas in summer minimum values down to 40~/o saturation are common in some deeper parts. Characteristic features of the plankton have been described by BAKKER (1964, 1972). The lake has a rich benthic fauna of worms, bivalves and crustaceans, particularly Corophium (VAAs, 1970).

III . DISTRIBUTION

Gobius niger is found along the entire coast of the Mediterranean and the Black Sea as well as along the Atlantic coast of France, the British Islands, Ireland and Norway till Trondheim. It occurs also in the western part of the Baltic (salinity 8 to 30 ~ S) till the Finnian Bight (6~/~ S), but not in the Botnian Gulf ( < 6~/~ S). Untill now the black goby has not been reported from the North Sea coast and estuaries of Belgium, the Netherlands, Germany and Denmark, although it is present along the eastern Baltic coast of Denmark. Many authors-- to be referred to later on- -ment ion its occurrence in brackish waters, but it has never been found in the brackish Dutch or German Wadden Sea. An obvious gap in the distribution pattern of this species is filled by the present occurrence of the black goby in the Veerse Meer.

Specimens from the Mediterranean have been described by some authors as a separate species, Gobius jozo. However, the differences between G. jozo and G. niger were reduced to changes induced by growth and sexual maturity by other authors (BORCEA, 1933; HASS, 1936; DE CASABLANCA & KIENER, 1969).

Gobi~ niger is a coastal fish able to live in brackish water down to 6~00 S. According to LEBOUR (1919) G. niger is common on sandy bottom off Plymouth, according to LE DANOIS ( 1913) fairly rare along the French Channel coast. Among the catches of numerous fishing trips in the Dutch coastal waters, carried out by the Netherlands Insti-

Page 6: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

G O B I U S N I G E R 59

tute of Sea Research, only a single black goby was found (personal communication M. Fonds). G. niger is often encountered between algal vegetation in sheltered areas. This holds particularly well for the Veerse Meer where the black goby is often found between threads of Chaetomorpha or fronds of Ulva and Callithamnion.

IV. MORPHOLOGICAL FEATURES OF GOBIUS NIGER FROM THE VEERSE MEER

The identification of Gi niger (Plate I) was elaborated with the aid of descriptions and illustrations from SANZO (1911), FAGV. (1914, 19154, 1915b), DE BUEN (1930), BORO~A (1933) and DE CASABIANCA & KIENER (1969). Special attention was paid to the position of the foramina and rows of dermal papillae on the head and front part of the body. Since the work of SnNzo these characteristics have been used to distinguish many genera and species of the European Gobiidae (SANZO, 1911; FAG~., 1915b; DE BUEN, 1930).

A study of more than 100 specimens yielded the following diagnosis:

D z = 4 ( 5 ) D , : 1 + 13 (12-14) A = 1 + 12 (13) Sq : (36--38) 39-40 (41)

The third through sixth ray of the first dorsal fin protrude in older males. The differences between the sexes, visible in the first dorsal fin and in the genital papillae, were the same as described by HOLT & BYRN~ (1898) and illustrated by MILLER (1961) for the closely related species Gobius paganellus.

The various ratios between the dimensions of the body of more than 50 fish measuring from 5 to 11 cm, are given in Fig. 2 as percentages of the body length. According to HASS (1936) the ratio between length and width, measured over a line just in front of the first dorsal and ventral fins, amounts to 6.2 for Mediterranean races and to 7.5 for

lOO

aos l ] I

Fig. 2. Average dimensions of Gobiuz niger of 5 to 11 cm in length in relation to total length.

Page 7: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

60 K . F . V A A S , A . G. V L A S B L O M & P . D E K O E I J E R

the nor th Atlant ic and Baltic races. In our mater ia l this rat io was 6.0. According to the same au tho r ano ther difference between nor the rn and southern races is the length of the longest r ay of the first dorsal fin in the males. I f this ray is ben t ove r backwards it will reach the 5th or 6th ray of the second dorsal fin in the M e d i t e r r a n e a n races and only the 2nd or 3rd ray of the second dorsal in the nor th Atlant ic races. We measured this character is t ic in 88 males longer than 8 cm. Th e longest dorsal ray of the first dorsal fin reached each of the 2nd th rough the 1 l th ray of the second dorsal fin and in a qua r t e r of the mater ia l it r eached be tween the 5th and 6th ray. This shows tha t the length of the dorsal fin is a r a the r var iable charac te r which does not indicate a dis t inct ion be tween groups or races with long and with short rays.

V. FOOD

Accord ing to DE CASABIANCA ~/; KIENER (1969) Gobius niger is a benthic feeder, ca tch ing its food by sight main ly dur ing the day. In ponds along the coast of Corsica benth ic molluscs and crustaceans proved to be the main food.

Food consumpt ion of black gobies collected in the Veerse Meer is summar ized in Tab le I. I t follows f rom the da ta tha t small G. niger differ f rom bigger ones in so far tha t they do not consume fish and consume a smaller quan t i t y of molluscs.

T A B L E I

Composition of stomach contents of 200 Gobius niger from the Veerse Meer in the course of 1971 to 1974. Differences between the diets of big and small fish tested

with the X 2 test for various combinations of items.

Length of Number out of 200fish with: X z p G. niger

(cm) Worms Crustaceans Molluscs Fish

< 6 174 86 36 0 > 6 160 92 64 16

22.64 < 0.001 0.66 > 0.05

141.68 < 0.001 7.89 < 0.05

In accordance wi th the food supply in the Veerse Mee r we recorded the following species of benthic fauna in the stomachs of G. niger: the worms Heteromastusfiliformis, Nerds diversicolor and various Nemer t in i ; the molluscs Mya arenaria, Cardium glaucum, Hydrobia spp. (mostly small individuals) ; the crustaceans Idotea chelipes, Praunusflexuosus and Neomysis

Page 8: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

OOBIUS NIGER 61

integer; the fish Pomatoschistus minutus and P. microps (small individuals). During winter the intestines were often empty, as was also observed in Corsica (DE CASABIANCA & KroNER, 1969).

VI. R E P R O D U C T I O N

The reproduction of the Mediterranean Gobius niger (G. jozo) was described by KINZER (1960), that of the related Atlantic species G. paganeUus by MILLER (1961). The reproduction of other related gobiid species has been described by NYMAN (1953), TAVOLGA (1954), JoNEs ~; MILLER (1966) and FONDS (1973). In our aquaria the black goby showed a similar behaviour as described for related gobiids. The male gobies dig holes under stones or shells and the females deposit their eggs on the underside of these hard substrates. The eggs are pear- shaped with a size of approximately 2 mm. In the Veerse Meer repro- duction of the black goby starts when the water temperature exceeds 12 ° C in May. According to various authors the reproduction period of G. niger may last from May till July-August (DuNCKER & LADIOES, 1960; Muus, 1966; DE CASABIANCA ~: KIENER, 1969). Ripe eggs are found in the Veerse Meer during April, May and the first part of June according to the weather circumstances, so that at the end of their first year the young gobies show a great variation in size. Black gobies which lived for some years in our aquaria with water supply from the Oosterschelde reproduced regularly every year in the course of May. At 12°C the eyes of the larvae are visible after about 12 days of development and they hatch after 20 days. Newly hatched larvae measure about 3 ram. They are characterized by a pigment star on the dorsal side and a row of 5 to 7 smaller spots on the ventral side from the anus backwards towards the end of the chorda (EHRENBAUM, 1905; PETERSEN, 1917, 1919; LEBOUR, 1919). Pelagic larvae of Gobius niger were caught frequently in the Veerse Meer. The smallest demersal specimens in the monthly catches (1 cm meshed trawl) measured 3.6 to 4 cm in September and October. These fish probably hatched in May.

V I I . G R O W T H AND L I F E SPAN; R E L A T I O N S H I P B E T W E E N W E I G H T AND L E N G T H

According to various authors the maximum age of Gobius niger may be 2 years (FAGE, 1915a, 1915b), 3 years (DuNcKER & LADIGES, 1960; MUUS, 1966) or even 4 to 5 years (MILLER, 1961). Some black gobies of 8 to 9 cm in length, caught in the Veerse Meer in December 1972, were kept in the aquarium in running water. They spawned in May

Page 9: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

6 c) K . F . VAAS, A. G. V L A S B L O M & P. DE K O E I J E R

1973, but failed to spawn once more in the spring of 1974. They died that same year at lengths of 15 to 16.7 cm, probably at an age of 3 to 4 years. Such large size was never observed in black gobies in the lake, which indicates that they show a lower growth rate or shorter life span in the lake.

The age of G. niger was estimated by counting year rings on the scales taken from the front part of the body just over the lateral line. Usually a number of scales had to be examined because sometimes a scale with one ring more or less was found.

The estimated length of different year classes of black gobies caught in the Veerse Meer during 1972 and 1973 is shown in Fig. 3. The numbers of gobies of one year class and one sex were often small or nil and amounted--the 19£8 year class excluded--to 20 fish at an average, sometimes more than 60 fish could be measured. A maximum of 5 year classes can be found together in the lake. The average length

length crn

14.

e~ e'68 13, • $8 e$e

12. 1~8 , ~ - - ' "

11.

10.

9. ~ s 8. 6.

5.

3.

o l j ' F ' M ' A ' . ' j ' J ' A ' S ' O ' N ' O I J ' F ' M ' A ' M ' J ' J ' A ' S ' O ' N ' OI J ' F ' 1972 1973 1974

Fig. 3. Growth of various year classes of Gobius niger in the Veerse Meer; 95% con- fidence limits indicated in those eases that more than 10 individuals were available.

, / j , /

'720' ~'d'. o-.--o. •

Page 10: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

GOBIUS N I G E R 63

from May to May, as read from the figure, amounts to 5 to 5.5 cm after one year, 8 to 9 cm after 2 years, 10 to 11 cm after 3 years and 12.5 to 13 cm after 4 years.

Fig. 3 also indicates that males are longer than females of the same age. An analysis of variance was carried out with the logarithmic values of length and weight of 1638 females and 1596 males captured in 26 fishing trips between 12 Janua ry 1971 and 16 January 1974 and classified per year class, making 38 monthly age groups. In 63% of these age groups the males were significantly longer than the females, and in 51% they were significantly longer as well as heavier. Within a monthly age group the males were never shorter or lighter than the females. Analyses of covariance (with adjustments for length), how- ever, showed that in 27% of all monthly age groups females were significantly heavier than males of the same length.

I f the logarithms of the length (L) are plotted against those of the weight (W) for both sexes, the growth curves run parallel in males and females. The overall formula for all animals caught in 3 years is:

y = 3 . 2 9 4 x - - 2.161 or W = 0.007L 3.~94

As the coefficient of L exceeds 3, growth of Gobius niger in the Veerse Mee t is not isometric.

After testing for aselectivity with Neumann's trend test (DE JONG% 1963; SACHS, 1971), the regression coefficient between length and weight of the animals in the populat ion--given in the above formula as an average over three years--appears to follow a cyclic course in t ime when calculated for every month separately (p < 0.05) (Fig. 4). Testing the difference between regression coefficients in winter and

b

:t 33.

3.1.

30.

23 .

1.8.

2.T.

2.6.

' ' F ' " ' A ' M ' J ' J ' * ' S ' ° ' " ' ° I J 'F ' " ' " ' " ' J ' J . m ~ ' " 's 'o ' " 'o lJ 'e ' " ' ' ' " 'J ,m" 'A 'S 'O ' " 'D IJ~ '

Fig. 4. Seasonal variation of the coefficient b in the formula W = aL b, estimated from the linear regression of log weight (IV) on log length (L), of Gobius niger caught

in the Veerse Meer during 1972 and 1973.

Page 11: Studies on the black Goby (Gobius Niger, Gobiidae, pisces) in the veerse meer, SW Netherlands

64 K. F. V A A S , A. G. V L A S B L O M & P. D E K O E I J E R

summer, showed significant differences between January 1971 and June 1971 (p < 0.001) and between February 1972 and August 1972 (p < 0.001). The factor b, depicted on the ordinate of Fig. 4, is virtually the condition factor of the fish. Its minimum in summer, when eggs have been shed and weight is relatively low, is clearly indicated in the figure. In winter the larger animals are heavier than those of" the same length in spring and summer.

MILLER (1964) found that adult Pomatoschistus microps start with a high growth rate in spring but slow down in growth during summer due to repeated spawning, the spawning process apparently annexing metabolic supplies otherwise involved in growth.

VIII . SEASONAL AND YEARLY F L U C T U A T I O N S IN NUMBERS

Fig. 5 shows the average number of black gobies caught per haul for each fishing trip. An estimation of the total population cannot be given before more is known about the gobies living in the shallow areas of the lake where our boat was unable to operate.

The numbers of gobies in the hauls varied considerably; in some

109.8

/ I

19. / '//?~1 l O

j 'F 'M'A 'M' j ' j 'A 'S 'O 'N 'DI j 'F 'M'A 'M' j ' j 'A 'S'O'N'DI j 'F 'M'A'M' j ' j 'A'S'O'N'DI 1971 1972 1973

Fig. 5. Number of Gobius niger per haul during 1971, 1972 and 1973, after log trans- formation, with 95% confidence limits. A haul took at an average 15 minutes and

covered an area of 3650 m2; number of hauls per trip 9 to 13.

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G O B I U S N I G E R 65

hauls none were caught whereas large numbers were caught in others. This shows that the distribution of the fish in the lake is not random. However, it is clear that the numbers increased in 1973. It is also evident that young fish enter the catchable stock in September- October. The numbers in the catches were at a minimum in January , February and March in all 3 years studied. According to JONES & MILLER (1966) and FONDS (1973) Pomatoschistus species leave the estuaries and migrate seawards in winter when temperature drops below 3 to 5 ° C. The black gobies in the Veerse Meer however, cannot emigrate in large numbers, because of the restricted connection be- tween the lake and the sea. For the isolated population the regularly observed decrease in numbers in winter means that the fish either hide under shells or stones or congregate at certain places not covered during the trawling surveys. These may be the shallow border areas of the lake, or, more likely, some deep spots in the former tidal gully.

IX. CHANGES IN THE DEMERSAL FISH FAUNA OF THE LAKE AFTER THE ESTABLISHMENT OF GOBIUS

NIGER

In spite of intensive fishing in the Veerse Meer from 1960 on, the first specimen of Gobius niger was only found in 1964, and it lasted until 1970 before a population of black gobies was established in the lake. Therefore, it can be assumed that we are dealing with a new intro- duction from the North Sea by way of the Oosterschelde.

The demersal fish fauna of the former open sea-arm was compared with that of the brackish lake by VAAS (1970). It is interesting to compare later changes of the fish fauna in the lake in the years after the black goby had made its appearance (Table II). Shortly after the enclosure of the Veerse Meer the number of fish species declined from about 34 in 1961 to about 17 in 1966. A slight reduction in the number of species took place in the following years, with the result that now only 12 species occur. Some common species that do not reproduce in the lake, like eel and plaice, showed a marked decline in numbers in the past years; the restricted recruitment of these fish by way of the sluices cannot match the fishing mortality (VAAs, 1970). Some other species as Atherina mochon and Gobius niger showed an increase in numbers. This indicates more favourable conditions for these species. The fact that the past 10 years were characterized by mild winters may be of considerable importance. Concomittant with an increase in the numbers of Gobius niger a decline of Pornatoschistus minutus is observed, while P. microps did not show any fall in numbers. This indicates that a certain amount of competition exists between the 2

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66 K . F . VAAS~ A. G. V L A S B L O M & P. D E K O E I J E R

TABLE II

Quantitative comparison of the fish fauna of the Veerse Meer in 4 different periods; the first one before enclosure. Numbers of fish calculated for a standard time of 700 minutes trawling, based on monthly fishing trips of 9 to 13 hauls per day.

(First 2 columns after V ~ s , 1970: 56.)

Species 3ul. 1960- Sep. 1965- Jan. 1972- jan. 1973- Apr. 1961 Oct. 1966 Dec. 1972 Dec. 1973

Pleuronectes platessa 825 880 23 13 Limanda limanda 21 41 0 0 Solea solea 41 5 1 0 Platichthys flesus 57 31 13 13 Odontogadus merlangus 5 1 0 0 Trisopterus luscus 94 0 0 0 Zoarces viviparus 98 155 61 65 Pholis gunnellus 68 1 0 0 Myxocephalus scorpius 73 26 0 0 Syngnathus rostellatus 218 2 0 3 Osmerus eperlanus 12 3 0 0 Sprattus sprattus 1300 340 4650 870 Ctupea harengus 120 3 285 8 Anguilla anguiUa 21 3190 495 113 Atherina presbyter 7 2 156 73 Atherina mochon 0 0 320 780 Ammodytes lancea 68 0 0 0 Pomatoschistus minutus 2540 4460 151 161 Pomatoschistus microps 288 1030 234 1430 Gobius niger 0 3 420 1170 Total number of

species, rare immi- grants included 34 17 16 12

first m e n t i o n e d species. Pomatoschistus microps is m u c h smal ler t han the 2 o the r gobies and also occupies a s o m e w h a t different niche: very shal low areas a n d ma in ly copepod food (DZ CASABIA~;CA & KtENER, 1969). Th is goby m a y therefore have escaped the i m p a c t o f the out- burs t o f the i m m i g r a n t Gobius niger. I n the same way, an increase in n u m b e r s of Atherina mochon took place a long wi th a decrease in n u m - bers of A. presbyter. I t will be of cons iderable interest to see w h a t will h a p p e n to the fish f auna of the l ake - -e spec ia l ly to the gobies and a t h e r i n i d s - - d u r i n g and after a severe winter .

X. S U M M A R Y

A descr ipt ion is g iven of the popu la t ion of Gobius niger, a fish new to the Du tch fauna, which establ ished itself in the brackish Veerse M e e r in the del ta a rea of the SW Nether lands . Some morpho log ic features

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GOBIUS NIGER 67

of the popula t ion are c o m p a r e d with those of populat ions elsewhere. Notes on feeding and rep roduc t ion are added. T h e growth of some year classes is described. I n the Veerse Meer 5 yea r classes were found together . A descript ion is given of changes in composi t ion and diversity o f the fish fauna of the area before and after enclosure o f the lake, at first appea rance of G. niger and after es tabl ishment of a large popu- lation.

XI. R E F E R E N C E S BAre,R, C., 1964. Planktonuntersuchungen in einem holl~tndlschcn Mecrc&arm vor

und nach der Abdeichung.--Helgol~inder wiss. Meeresunters. 10: 456--471. , 1972. Milieu en plankton van het Veerse Mccr, een tien jaar oud brakwater meer in zuidoost Ncderland.--Medcd. Ned. Hydr. Ver. 6:15-38 (Eng. suture.).

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