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Environmental constraints on the sand lance population in the eastern Seto Inland Sea
Environmental Constraints on the Sand Lance Population in the Eastern Seto Inland Sea H. NAKATA*, S. KIMURA*, M. J. K1SHI* and ~ FUJIWARAt *Ocean Research Nstituw, Unive~i~ of Tokyo, 1-~-1 MinamidaL Nakano, Tokyo, ~4 Japan t Government Industrial Research Insgtuw, Chugoku, Hiro-suehiro, Ku~, Hiroshima, ~ Japan I n t h i s p a p e r , f o c u s i n g on t h e r e c r u i t m e n t p r o c e s s e s , we s y n t h e s i z e the r e s u l t s o f our r e c e n t a n a l y t i c a l and n u m e r i c a l s t u d i e s on t h e p o p u l a t i o n e c o l o g y o f t h e J a p a n e s e sand l a n c e i n t h e e a s t e r n p a r t o f t h e S e t o InIand Sea. Among p h y s i c a l p r o c e s s e s , t h e w i n d - i n d u c e d c u r r e n t has a s i g n i f i c a n t e f f e c t on t h e t r a n s p o r t and d i s t r i b u t i o n o f t h e l a r v a e . Prey-predator interactions, competion for the specific habitats, and o t h e r e c o l o g i c a l processes could affect the recruitment. In a d d i t i o n t o t h e y e a r t o y e a r variability o f t h e l a r v a l h a t c h i n g and apparent s u r v i v a l r a t e s , some r e c e n t trends of the variation in the larval population are also discussed. The Japanese sand lance Ammodytes personatus Girard is one of the commercially important fish in the coastal waters of Japan. In the eastern part of the Seto Inland Sea (Bisan Strait, Sea of Harima and Osaka Bay, Fig.l), its annual catch amounts to approximately 39,000 tons (average from 19741984), which accounts for about 22% of the total fisheries production in this area. It is also known that the catch varies considerably due to the recruitment success or failure of the O-age fish. Thus the environmental factors related to the survival of the sand lance larvae have become a matter of great importance for catch forecasting. In this paper, focusing on the recruitment processes of the sand lance population in the eastern Seto Inland Sea, we synthesize the results of our recent analytical and numerical studies. Some recent trends of the variation in the larval population are also discussed. Environmental f a c t o r s r e l a t e d to the sand lance recruitment The main spawning g r o u n d s o f t h e sand l a n c e in t h e e a s t e r n S e t o I n l a n d Sea a r e l o c a t e d in t h e c e n t r a l Bisan S t r a i t and S h i k a n o - S e i n t h e n o r t h e a s t o f t h e Sea o f Harima ( F i g . l ) . The main spawning p e r i o d i s i n w i n t e r ( e a r l y December-early January). The eggs a d h e r e t o bottom sand f o r a b o u t 25 d a y s , t h e n h a t c h o u t and s p e n d a b o u t 2 month p e l a g i c p e r i o d . The O - a g e f i s h (young) become a t a r g e t o f t h e f i s h e r y from l a t e March t o l a t e J u n e . After this they estivate in the bottom sand until early December, when they are matured and start to spawn (Hamada, 1985). Various environmental factors including natural and human impacts related to the recruitment of the sand lance population are summarized in Fig.2. Since strong westerly wind prevails during the main spawning 131"E 133" 135" period, wind-induced cur! / i ! ! r e n t has a s i g n i f i c a n t Kurushlma Str, Blsan S t r . e f f e c t on t h e t r a n s p o r t -~-~_. \ BINGO \ HARI~IA SIkanose SUOU . - - - . \ GEIYO .~\ ~ / Akoshl St/, and d i s t r i b u t i o n o f t h e ~'N_ pelagic larvae. In a d d i t i o n , some o c e a n i c c o n d i t i o n s such as w a t e r t e m p e r a t u r e and s a l i n i t y , - 3~4"N and e c o l o g i c a l p r o c e s s e s including prey-predator interactions could affect t h e l a r v a l growth and 133"E 135" r e c r u i t m e n t ; however t h e r e Fig.l The S e t o I n l a n d Sea, Shaded areas i s so f a r l i t t l e q u a n t i t a t i v e indicate the spawning grounds of sand lance. i n f o r m a t i o n a b o u t them. Marine Pollution Bulletin, Vol. 23. pp. 195-199, 1991, Printed in Great Britain 0025-326X/91 S3.00+0.011 © 1991 Pergamon Press plc
195
Marine Pollution Bulletin
Besides the natural impacts mentioned above, the fishery itself could have a serious impact upon the population. In fact, the apparent increase in the sand lance catch during 1960s was partly due to the development of fishing gears, corresponding with a growing demand (Hamada, 1985). Further, the habitats during both spawning and estivation periods are confined to specific sandy banks. This may possibly determine the carrying capacity of the population; therefore, conservation of the sandy banks against human impacts is essential to the resource management of sand lance. A n a l y t i c a l and n u m e r i c a l approaches t o t h e r e c r u i t m e n t dynamics Wind-induced l a r v a l t r a n s p o r t Hamada(1966) f i r s t r e p o r t e d t h a t t h e r e i s a p o s i t i v e correlation between t h e f r e q u e n c y o f t h e s t r o n g w e s t e r l y wind a f t e r t h e p e a k s p a w n i n g and t h e c a t c h o f 0 - a g e f i s h i n t h e Sea o f Harima. N a k a t a ( 1 9 8 8 ) f u r t h e r showed t h a t w i n d - i n d u c e d e a s t w a r d t r a n s p o r t o f t h e l a r v a e from t h e B i s a n S t r a i t c o n t r i b u t e s t o t h e c a t c h i n c r e a s e in t h e Sea o f Harima. F u j i w a r a e t a l . (1990) have r e c e n t l y p r o v i d e d a q u a n t i t a t i v e basis for the discussion. According to their estimates ( a v e r a g e s from 1981-1986) a b o u t 65% o f t h e h a t c h e d l a r v a e in t h e e a s t e r n S e t o I n l a n d Sea o r i g i n a t e from t h e B i s a n S t r a i t , and a b o u t 86% o f t h e h a t c h e d l a r v a e o f t h e B i s a n S t r a i t o r i g i n a r e t r a n s p o r t e d i n t o t h e Sea o f Harima from J a n u a r y - F e b r u a r y . This transport rate is consistent w i t h t h e w i n d - i n d u c e d e a s t w a r d volume transport ( a v e r a g e 4 , 0 0 0 m 3 / s ) e s t i m a t e d by F u j i w a r a and Higo {1986). The above results support the hypothesis previously presented for the catch f o r e c a s t , and g i v e a m e c h a n i s t i c b a s i s f o r i t .
Larval population dynamics Fig.3 shows the year to year variations in the number of sand lance larvae collected in February. I t is noticeable that large year classes in the Bisan S t r a i t occurred at i n t e r v a l s of 5-6 years. In s p i t e of the appearance of a large 1989 year class, there is a significant decline in the larval density in the Bisan Strait. Somepeaks appearing in the Sea of Harima coincide with those in the Bisan S t r a i t (1973, 1989, e t c . ) . The larval density in Osaka Bay (not shown) has rapidly increased since 1985.
ECOLOGICAL SYSTEMS NAT URAL P-O-P~rJ-L'AT~O--N'-" DYNAM I C S
l I
NATURAL IMPACTS
i i I r
!0_J
LARVAL
"i"::'"" + " ,IPREYDENSITy~L~I~EEDING 'AT+ ~+Jl poE ..... .... ,+>x._+r
HUMAN IMPACTS~ FISHERY
.i+..+i% SPAWNING
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, TOPOGRAPHY}:'i]~-----'I'---~" +.-+-""+~I
Fig.2
Environmental
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196
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of
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__J
Volume23/1991/EMECS"90 indi./haul
140
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'
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number o f t h e l a r v a e of the survival rate),
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Based on t h e a v e r a g e s o f t h e l a r v a l d e n s i t y from 1 9 8 1 - 1 9 8 6 , Fujiwara e t al. ( 1 9 9 0 ) p r o p o s e d an a n a l y t i c a l method for determining biological p a r a m e t e r s s u c h as h a t c h i n g i n t e n s i t y and s u r v i v a l r a t e o f t h e l a r v a e . They c o n s i d e r e d t h e e a s t e r n S e t o I n l a n d Sea as an e c o l o g i c a l l y semi-closed a r e a f o r t h e sand l a n c e p o p u l a t i o n , and a n a l y z e d t h e time change o f t h e l a r v a l d e n s i t y and age c o m p o s i t i o n in t h i s a r e a from January t o February. The t o t a l number o f hatched l a r v a e e s t i m a t e d by t h i s method i s 2 . 7 6 × 1012 w i t h maximal h a t c h i n g i n t e n s i t y o f 6 . 8 6 × 10 l ° i n d . / d a y . The s u r v i v a l r a t e f u n c t i o n i s , on t h e o t h e r hand, g i v e n as t h e e x p o n e n t i a l function with diminishing coefficient o f 0 . 0 7 1 / d a y , showing an apparent monthly s u r v i v a l rate is 0.119. Using t h e same method as Fujiwara e t a l . ( 1 9 9 0 ) , we l o o k a t t h e i n t e r a n nual changes o f t h e number o f h a t c h e d l a r v a e and t h e d i m i n i s h i n g c o e f f i c i e n t o f t h e l a r v a l s u r v i v a l f u n c t i o n in t h e Bisan S t r a i t from 1966 t o 1988 ( e x c e p t f o r 1969 and 1974 b e c a u s e o f i n c o m p l e t e d a t a s e t ) . The r e s u l t s o f t h e a n a l y s i s are shown w i t h t h e n o r m a l i z e d v a l u e s i n F i g . 4 . Extremely high hatching intensity, against high diminishing coefficient, led to l a r g e s t 1973 year c l a s s , w h i l e h i g h e r abundance o f h a t c h e d l a r v a e d i d n o t r e s u l t in l a r g e year c l a s s e s in 1970 and 1977 due t o poor c o n d i t i o n s o f survival (and/or transport loss). For 1978, t h e t o t a l number o f h a t c h e d l a r v a e was below a v e r a g e , but l a r g e year c l a s s r e s u l t e d from some f a v o r a b l e survival conditions. It is also noted that higher hatching intensity often corresponds to higher diminishing coefficient; t h i s nay s u g g e s t a d e n s i t y dependent m o r t a l i t y . The l a r v a l d e n s i t y s i m u l a t e d ( F i g . 4 c ) by u s i n g t h e hatching intensity and s u r v i v a l r a t e f u n c t i o n s s u c c e s s f u l l y a c c o u n t s f o r the observed data {r=0.93). The d i m i n i s h i n g c o e f f i c i e n t (apparent s u r v i v a l r a t e ) shown in F i g . 4 i n c l u d e s a d v e c t i v e l o s s due t o w i n d - i n d u c e d l a r v a l t r a n s p o r t from t h e Bisan S t r a i t t o t h e Sea o f Hariaa. We t h e r e f o r e p l o t t e d t h e d e v i a t i o n s from t h e mean in a wind i n d e x - d i m i n i s h i n g c o e f f i c i e n t diagram ( F i g . 5 ) . The wind i n dex (WI) i s an a p p r o p r i a t e i n d i c a t o r o f t h e w e s t e r l y monsoon i n w i n t e r ( h i g h v a l u e o f WI c o r r e l a t e s w i t h s t r o n g w e s t e r l y w i n d ) ( F u j i w a r a , 1990). There i s a p p a r e n t l y no c o r r e l a t i o n between variations of the diminishing coefficient and WI. This c o u l d be p r o b a b l y due t o some d e n s i t y dependent 197
Marine Pollution Bulletin
p r o c e s s e s (1970, 1972, 1973 and 1983) and f a v o r a b l e survival conditions despite the wind-induced transport l o s s (1967, 1978, 1981 and 1985), which make the correlation unclear, It should be noted in Fig.5 that high diminishing coefficients obtained in 1987 and 1988 were not resulted from wind-induced transport loss; some other unknown factors could cause poor conditions for larval survival in a d d i t i o n t o t h e low h a t c h ing i n t e n s i t y ( F i g . 4 ) in recent years•
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the calendar years.
A biomass-based ecosystem model of the sand lance population in the eastern Seto Inland Sea has been developed by Kishi et al• (1990)• The following six state variables were taken into the model: eggs, larvae, young, oneyear old and two(or more)-year old adults of the sand lance, and zooplankton. The transition between stages of the sand lance is given according to calendar date, and the zooplankton biomass is assumed to be a function of water temperature. The emphasis is placed upon a numerical balance of the model parameters to maintain a steady population rather than a numerical simulation of actual dynamics. Physical processes such as wind-induced larval transport has not been included in the model. According to the estimate based on this model, the mortality rate of estivating young could be higher than that previously expected. This suggests the importance of the ecological study on the estivating young to detect the factors crucial to the mortality during the estivation period. In addition, the sensitivity analysis of the model shows that the biomass of zooplankton could play an important role in the variation of the sand lance abundance. In this model, the zooplankton functions as a predator against the sand lance larvae besides functioning as its prey; this complicate the interpretation of the results. Further elaboration of a sophisticated ecosystem model which takes actual prey-predator interactions into account is necessary.
Concluding remarks As d e s c r i b e d above, r e c e n t a n a l y t i c a l s t u d i e s have q u a n t i f i e d w i n d - i n d u c e d transport, hatching intensity, and a p p a r e n t s u r v i v a l r a t e o f t h e l a r v a l population. In t h e B i s a n S t r a i t , high hatching i n t e n s i t y did not always l e a d t o l a r v a l abundance ( F i g . 4 ) . The t r a n s p o r t l o s s due t o t h e w e s t e r l y w i n d was n o t t h e o n l y f a c t o r determining the apparent survival rate ( F i g . 5 ) ; t h e r e a r e s t i l l u n c e r t a i n t i e s i n t h e mechanism o f t h e v a r i a b i l i t y of the survival rate. Due t o low h a t c h i n g i n t e n s i t y and r e l a t i v e l y h i g h diminishing coefficient ( p r o b a b l y h i g h m o r t a l i t y ) , t h e l a r v a l abundance i n t h e B i s a n S t r a i t has d e c l i n e d in r e c e n t y e a r s ( F i g . 4 ) . The s u b s e q u e n t c a t c h o f 0 - a g e f i s h a l s o shows a d e c r e a s i n g t r e n d ( n o t shown)• Human a c t i v i t i e s such as sand dredging in the spawning grounds could affect the larval h a t c h i n g and t h e s u r v i v a l r a t e d u r i n g p o s t - l a r v a l period. In a d d i t i o n , among p h y s i c a l p r o p e r t i e s s a l i n i t y shows p o s i t i v e d e v i a t i o n s from 1984-1988 ( n o t shown). F u r t h e r i n v e s t i g a t i o n i s r e q u i r e d on t h e c a u s a l f a c t o r s o f r e c e n t r e d u c t i o n i n t h e l a r v a l d e n s i t y i n t h e Bisan S t r a i t • On t h e o t h e r hand, t h e w i n d - i n d u c e d t r a n s p o r t o f t h e l a r v a e o r i g i n a t i n g from t h e Bisan S t r a i t c o n t r i b u t e s t o t h e l a r v a l abundance i n t h e Sea o f Harima. F i g . 6 f u r t h e r shows t h e r e l a t i o n between t h e l a r v a l abundance and the subsequent catch of 0-age fish. I t i s e v i d e n t i n t h e Sea o f Harima 198
Volume 2 3 / 1 9 9 1 / E M E C S '90
that the larval abunS E A OF H A R I M A dance in February is 50000 closely correlated • 70 with the catch during 1960s and 1970s. C ,o00o The corrrelation coefficient is 0.78 except f o r t h e d a t a from 1973, ~ 30000 065 ,75 / when the larval hatch• 74 059 o 67 X81 ing was extremely large.~ °72 / ~1Cj79 However, during 1980s ~ 20000 8Ox X 8 7 / e75 x 82 e s p e c i a l l y from 1985 < TT, x 83 0 6 6 • 73 x85 when t h e l a r v a l d e n s i t y /X e78 6 4 0 8-4 x86 x 88 in Osaka Bay rapidly 10000 , / O60 062 increased, the catch in the Sea of Harima 20 40 60 80 100 seemed to decline in N u m b e r Of l a r v a e collected in F e b r u a r y ( ind. / haul} spite of rather high larval density. The Fig.6 Relation between larval abundance in February and the catches of 0-age fish subsequent catch of the 0-age fish in the Sea of Harima. in the Bisan Strait The data from 1960s, 1970s and 1980s are plotted with dots, (not shown) were rather open circles and crosses, respectively. small when the larval densities in February were over 50 (ind./haul). These imply existence of density dependent processes in the post-larval period• We have also referred to a density dependent mortality during the larval period (Fig.4). These density dependent processes possibly include competition for the prey and the habitats (i.e. sand banks). The larval mortality due to cannibalism (adult predation on the larvae) is also a possible mechanism (Hamada,1985). A more detailed field survey of the above ecological problems and ecosystem modelling of the prey-predator interactions would be enlightening. The data obtained from long-term monitoring by the Hyogo and Okayama Prefectural Fisheries Experimental Stations enable us to carry out the analyses presented above. We would emphasize that the continuous monitoring, combined with well-designed experiments (including numerical studies), holds the key to unveiling the dynamic behaviours of the larval population in recent years. Acknowledgements: We thank Ms.A.Kuramoto for her help with the preparation of the manuscript, and Dr.M.K•Antony for correcting the English. ~eferences Fujiwara, T. (1990}: Variation in the catch of sand eel related to the climate and sea conditions. Umi to Sofa (in press). Fujiwara, T. and T. Higo (1966): Wind-induced current and mass transport in the Seto Inland Sea. Rull. Coast.Oceanogr., 23, 109-119. Fujiwara, T., H.Nakata, M.Tanda and J.Karakawa (1990): Biological and physical parameters of the population dynamics of sand eel larvae in the eastern Seto Inland Sea.
Nippon Suisan Gakkaishi, 56(in press).
Hamada, T. (1966): Studies on fluctuation in the abundance of larval s a ~ - I m n c e
in
the Harima-Nada and Osaka Bay. [[[. Relationship to weather and sea conditions during the breeding season.
Bull.Jap. Soc. Sci. Fish., 32,579-584.
Hamada, T. (1985): Fishery Biology of the Sand-lance ( ~ o d y t e s personatus Girard) in Japan.
Japan Fisheries Resource Conservation Association, Tokyo, 1985, 82pp.
Kishi, M.J., S.Kimura, H.Nakata and Y.Yamashita (1990}: A biomass-based model for the sand lance (Ammodytes personatus) in Seto Inland-sea (Japan). Ecol. Modelling (in press). Nakata, H.(1988): Wind effects on the transport of Japanese sand eel larvae in the eastern part of the Seto Inland Sea.
Nippon Suis~In Cakkaishi,
54, 1553-1561.
199
195
Marine Pollution Bulletin
Besides the natural impacts mentioned above, the fishery itself could have a serious impact upon the population. In fact, the apparent increase in the sand lance catch during 1960s was partly due to the development of fishing gears, corresponding with a growing demand (Hamada, 1985). Further, the habitats during both spawning and estivation periods are confined to specific sandy banks. This may possibly determine the carrying capacity of the population; therefore, conservation of the sandy banks against human impacts is essential to the resource management of sand lance. A n a l y t i c a l and n u m e r i c a l approaches t o t h e r e c r u i t m e n t dynamics Wind-induced l a r v a l t r a n s p o r t Hamada(1966) f i r s t r e p o r t e d t h a t t h e r e i s a p o s i t i v e correlation between t h e f r e q u e n c y o f t h e s t r o n g w e s t e r l y wind a f t e r t h e p e a k s p a w n i n g and t h e c a t c h o f 0 - a g e f i s h i n t h e Sea o f Harima. N a k a t a ( 1 9 8 8 ) f u r t h e r showed t h a t w i n d - i n d u c e d e a s t w a r d t r a n s p o r t o f t h e l a r v a e from t h e B i s a n S t r a i t c o n t r i b u t e s t o t h e c a t c h i n c r e a s e in t h e Sea o f Harima. F u j i w a r a e t a l . (1990) have r e c e n t l y p r o v i d e d a q u a n t i t a t i v e basis for the discussion. According to their estimates ( a v e r a g e s from 1981-1986) a b o u t 65% o f t h e h a t c h e d l a r v a e in t h e e a s t e r n S e t o I n l a n d Sea o r i g i n a t e from t h e B i s a n S t r a i t , and a b o u t 86% o f t h e h a t c h e d l a r v a e o f t h e B i s a n S t r a i t o r i g i n a r e t r a n s p o r t e d i n t o t h e Sea o f Harima from J a n u a r y - F e b r u a r y . This transport rate is consistent w i t h t h e w i n d - i n d u c e d e a s t w a r d volume transport ( a v e r a g e 4 , 0 0 0 m 3 / s ) e s t i m a t e d by F u j i w a r a and Higo {1986). The above results support the hypothesis previously presented for the catch f o r e c a s t , and g i v e a m e c h a n i s t i c b a s i s f o r i t .
Larval population dynamics Fig.3 shows the year to year variations in the number of sand lance larvae collected in February. I t is noticeable that large year classes in the Bisan S t r a i t occurred at i n t e r v a l s of 5-6 years. In s p i t e of the appearance of a large 1989 year class, there is a significant decline in the larval density in the Bisan Strait. Somepeaks appearing in the Sea of Harima coincide with those in the Bisan S t r a i t (1973, 1989, e t c . ) . The larval density in Osaka Bay (not shown) has rapidly increased since 1985.
ECOLOGICAL SYSTEMS NAT URAL P-O-P~rJ-L'AT~O--N'-" DYNAM I C S
l I
NATURAL IMPACTS
i i I r
!0_J
LARVAL
"i"::'"" + " ,IPREYDENSITy~L~I~EEDING 'AT+ ~+Jl poE ..... .... ,+>x._+r
HUMAN IMPACTS~ FISHERY
.i+..+i% SPAWNING
TOPOGRAPHy "
, TOPOGRAPHY}:'i]~-----'I'---~" +.-+-""+~I
Fig.2
Environmental
lance population
196
......
factors
related
in t h e e a s t e r n
i'.!;;:.:...'.':.,:./ I;"':~~
to t h e r e c r u i t m e n t
Seto
I n l a n d Sea.
of
the sand
__J
Volume23/1991/EMECS"90 indi./haul
140
(a)
120 100
BISAN
NUMBER
STRAIT
HATCHED
OF LARVAE
(ESTIMATED)
150 4O 2O 0 65
"7 .,~ LU ~E
~.~.,,,,i 70 75
. . . .
i .... I,,,,I 80 85 90
140 120
SEA
0 rn .~
0
~ c o
-I 2
- APPARENT
~,(
,
MONTHLY A T E
,;, ~// SURVIVAL,,/'~R
(b)
DIMINISHING COEFFICIENT
,u "~ 0 "o
OF
(ESTIMATED) 7 0
100
.~ c
80
(c) "¢
6O
2
NUMBER
4O
OF
LA RVAE COLLECTED
20 O 0
.... 65
i .... ,..,i .... i .... r 70 75 80 85 9 0 YEAR
-I -c,
IN FEBRUARY
I
N" F'
"
'
"
1970
Fig.3
Year t o y e a r v a r i a t i o n
Fig.4
"
'
,
1g75
'
"
I
0 J
1 9 8 0
Normarized values
~ •
of
(a) total
hatched,
(b) dJJsinishing coefficient
larvae
function
(apparent monthly survival
in February.
density
observed
o SIMULATED
•
1 9 9 5
in the abundance of sand lance collected
o....
number o f t h e l a r v a e of the survival rate),
and ( c )
rate larval
in February in the Btsan Strait.
Based on t h e a v e r a g e s o f t h e l a r v a l d e n s i t y from 1 9 8 1 - 1 9 8 6 , Fujiwara e t al. ( 1 9 9 0 ) p r o p o s e d an a n a l y t i c a l method for determining biological p a r a m e t e r s s u c h as h a t c h i n g i n t e n s i t y and s u r v i v a l r a t e o f t h e l a r v a e . They c o n s i d e r e d t h e e a s t e r n S e t o I n l a n d Sea as an e c o l o g i c a l l y semi-closed a r e a f o r t h e sand l a n c e p o p u l a t i o n , and a n a l y z e d t h e time change o f t h e l a r v a l d e n s i t y and age c o m p o s i t i o n in t h i s a r e a from January t o February. The t o t a l number o f hatched l a r v a e e s t i m a t e d by t h i s method i s 2 . 7 6 × 1012 w i t h maximal h a t c h i n g i n t e n s i t y o f 6 . 8 6 × 10 l ° i n d . / d a y . The s u r v i v a l r a t e f u n c t i o n i s , on t h e o t h e r hand, g i v e n as t h e e x p o n e n t i a l function with diminishing coefficient o f 0 . 0 7 1 / d a y , showing an apparent monthly s u r v i v a l rate is 0.119. Using t h e same method as Fujiwara e t a l . ( 1 9 9 0 ) , we l o o k a t t h e i n t e r a n nual changes o f t h e number o f h a t c h e d l a r v a e and t h e d i m i n i s h i n g c o e f f i c i e n t o f t h e l a r v a l s u r v i v a l f u n c t i o n in t h e Bisan S t r a i t from 1966 t o 1988 ( e x c e p t f o r 1969 and 1974 b e c a u s e o f i n c o m p l e t e d a t a s e t ) . The r e s u l t s o f t h e a n a l y s i s are shown w i t h t h e n o r m a l i z e d v a l u e s i n F i g . 4 . Extremely high hatching intensity, against high diminishing coefficient, led to l a r g e s t 1973 year c l a s s , w h i l e h i g h e r abundance o f h a t c h e d l a r v a e d i d n o t r e s u l t in l a r g e year c l a s s e s in 1970 and 1977 due t o poor c o n d i t i o n s o f survival (and/or transport loss). For 1978, t h e t o t a l number o f h a t c h e d l a r v a e was below a v e r a g e , but l a r g e year c l a s s r e s u l t e d from some f a v o r a b l e survival conditions. It is also noted that higher hatching intensity often corresponds to higher diminishing coefficient; t h i s nay s u g g e s t a d e n s i t y dependent m o r t a l i t y . The l a r v a l d e n s i t y s i m u l a t e d ( F i g . 4 c ) by u s i n g t h e hatching intensity and s u r v i v a l r a t e f u n c t i o n s s u c c e s s f u l l y a c c o u n t s f o r the observed data {r=0.93). The d i m i n i s h i n g c o e f f i c i e n t (apparent s u r v i v a l r a t e ) shown in F i g . 4 i n c l u d e s a d v e c t i v e l o s s due t o w i n d - i n d u c e d l a r v a l t r a n s p o r t from t h e Bisan S t r a i t t o t h e Sea o f Hariaa. We t h e r e f o r e p l o t t e d t h e d e v i a t i o n s from t h e mean in a wind i n d e x - d i m i n i s h i n g c o e f f i c i e n t diagram ( F i g . 5 ) . The wind i n dex (WI) i s an a p p r o p r i a t e i n d i c a t o r o f t h e w e s t e r l y monsoon i n w i n t e r ( h i g h v a l u e o f WI c o r r e l a t e s w i t h s t r o n g w e s t e r l y w i n d ) ( F u j i w a r a , 1990). There i s a p p a r e n t l y no c o r r e l a t i o n between variations of the diminishing coefficient and WI. This c o u l d be p r o b a b l y due t o some d e n s i t y dependent 197
Marine Pollution Bulletin
p r o c e s s e s (1970, 1972, 1973 and 1983) and f a v o r a b l e survival conditions despite the wind-induced transport l o s s (1967, 1978, 1981 and 1985), which make the correlation unclear, It should be noted in Fig.5 that high diminishing coefficients obtained in 1987 and 1988 were not resulted from wind-induced transport loss; some other unknown factors could cause poor conditions for larval survival in a d d i t i o n t o t h e low h a t c h ing i n t e n s i t y ( F i g . 4 ) in recent years•
2 t'
z ~
• 88
t
87-1
e&8 I I
o82
~83
w o
I • 72
$6"
i
-14
I
o
. . . . . . . . . . .
f-- . . . . . . .
z ~ -i z
t
"75
• 71 I
"80
;7)
t
• 66
85"
I
- - -
.al
,67
i
•78
I t
-2
1 -2
-I
W Fig.5
Relation
in diminishing
Ecosxstem model I ink
t 1 i
.73
W
~o
Feb.).
0
I
IN D
IN D E X
between
the variations
coefficient
Numerals
indicate
and wind
(in SD) index (Jan.-
the calendar years.
A biomass-based ecosystem model of the sand lance population in the eastern Seto Inland Sea has been developed by Kishi et al• (1990)• The following six state variables were taken into the model: eggs, larvae, young, oneyear old and two(or more)-year old adults of the sand lance, and zooplankton. The transition between stages of the sand lance is given according to calendar date, and the zooplankton biomass is assumed to be a function of water temperature. The emphasis is placed upon a numerical balance of the model parameters to maintain a steady population rather than a numerical simulation of actual dynamics. Physical processes such as wind-induced larval transport has not been included in the model. According to the estimate based on this model, the mortality rate of estivating young could be higher than that previously expected. This suggests the importance of the ecological study on the estivating young to detect the factors crucial to the mortality during the estivation period. In addition, the sensitivity analysis of the model shows that the biomass of zooplankton could play an important role in the variation of the sand lance abundance. In this model, the zooplankton functions as a predator against the sand lance larvae besides functioning as its prey; this complicate the interpretation of the results. Further elaboration of a sophisticated ecosystem model which takes actual prey-predator interactions into account is necessary.
Concluding remarks As d e s c r i b e d above, r e c e n t a n a l y t i c a l s t u d i e s have q u a n t i f i e d w i n d - i n d u c e d transport, hatching intensity, and a p p a r e n t s u r v i v a l r a t e o f t h e l a r v a l population. In t h e B i s a n S t r a i t , high hatching i n t e n s i t y did not always l e a d t o l a r v a l abundance ( F i g . 4 ) . The t r a n s p o r t l o s s due t o t h e w e s t e r l y w i n d was n o t t h e o n l y f a c t o r determining the apparent survival rate ( F i g . 5 ) ; t h e r e a r e s t i l l u n c e r t a i n t i e s i n t h e mechanism o f t h e v a r i a b i l i t y of the survival rate. Due t o low h a t c h i n g i n t e n s i t y and r e l a t i v e l y h i g h diminishing coefficient ( p r o b a b l y h i g h m o r t a l i t y ) , t h e l a r v a l abundance i n t h e B i s a n S t r a i t has d e c l i n e d in r e c e n t y e a r s ( F i g . 4 ) . The s u b s e q u e n t c a t c h o f 0 - a g e f i s h a l s o shows a d e c r e a s i n g t r e n d ( n o t shown)• Human a c t i v i t i e s such as sand dredging in the spawning grounds could affect the larval h a t c h i n g and t h e s u r v i v a l r a t e d u r i n g p o s t - l a r v a l period. In a d d i t i o n , among p h y s i c a l p r o p e r t i e s s a l i n i t y shows p o s i t i v e d e v i a t i o n s from 1984-1988 ( n o t shown). F u r t h e r i n v e s t i g a t i o n i s r e q u i r e d on t h e c a u s a l f a c t o r s o f r e c e n t r e d u c t i o n i n t h e l a r v a l d e n s i t y i n t h e Bisan S t r a i t • On t h e o t h e r hand, t h e w i n d - i n d u c e d t r a n s p o r t o f t h e l a r v a e o r i g i n a t i n g from t h e Bisan S t r a i t c o n t r i b u t e s t o t h e l a r v a l abundance i n t h e Sea o f Harima. F i g . 6 f u r t h e r shows t h e r e l a t i o n between t h e l a r v a l abundance and the subsequent catch of 0-age fish. I t i s e v i d e n t i n t h e Sea o f Harima 198
Volume 2 3 / 1 9 9 1 / E M E C S '90
that the larval abunS E A OF H A R I M A dance in February is 50000 closely correlated • 70 with the catch during 1960s and 1970s. C ,o00o The corrrelation coefficient is 0.78 except f o r t h e d a t a from 1973, ~ 30000 065 ,75 / when the larval hatch• 74 059 o 67 X81 ing was extremely large.~ °72 / ~1Cj79 However, during 1980s ~ 20000 8Ox X 8 7 / e75 x 82 e s p e c i a l l y from 1985 < TT, x 83 0 6 6 • 73 x85 when t h e l a r v a l d e n s i t y /X e78 6 4 0 8-4 x86 x 88 in Osaka Bay rapidly 10000 , / O60 062 increased, the catch in the Sea of Harima 20 40 60 80 100 seemed to decline in N u m b e r Of l a r v a e collected in F e b r u a r y ( ind. / haul} spite of rather high larval density. The Fig.6 Relation between larval abundance in February and the catches of 0-age fish subsequent catch of the 0-age fish in the Sea of Harima. in the Bisan Strait The data from 1960s, 1970s and 1980s are plotted with dots, (not shown) were rather open circles and crosses, respectively. small when the larval densities in February were over 50 (ind./haul). These imply existence of density dependent processes in the post-larval period• We have also referred to a density dependent mortality during the larval period (Fig.4). These density dependent processes possibly include competition for the prey and the habitats (i.e. sand banks). The larval mortality due to cannibalism (adult predation on the larvae) is also a possible mechanism (Hamada,1985). A more detailed field survey of the above ecological problems and ecosystem modelling of the prey-predator interactions would be enlightening. The data obtained from long-term monitoring by the Hyogo and Okayama Prefectural Fisheries Experimental Stations enable us to carry out the analyses presented above. We would emphasize that the continuous monitoring, combined with well-designed experiments (including numerical studies), holds the key to unveiling the dynamic behaviours of the larval population in recent years. Acknowledgements: We thank Ms.A.Kuramoto for her help with the preparation of the manuscript, and Dr.M.K•Antony for correcting the English. ~eferences Fujiwara, T. (1990}: Variation in the catch of sand eel related to the climate and sea conditions. Umi to Sofa (in press). Fujiwara, T. and T. Higo (1966): Wind-induced current and mass transport in the Seto Inland Sea. Rull. Coast.Oceanogr., 23, 109-119. Fujiwara, T., H.Nakata, M.Tanda and J.Karakawa (1990): Biological and physical parameters of the population dynamics of sand eel larvae in the eastern Seto Inland Sea.
Nippon Suisan Gakkaishi, 56(in press).
Hamada, T. (1966): Studies on fluctuation in the abundance of larval s a ~ - I m n c e
in
the Harima-Nada and Osaka Bay. [[[. Relationship to weather and sea conditions during the breeding season.
Bull.Jap. Soc. Sci. Fish., 32,579-584.
Hamada, T. (1985): Fishery Biology of the Sand-lance ( ~ o d y t e s personatus Girard) in Japan.
Japan Fisheries Resource Conservation Association, Tokyo, 1985, 82pp.
Kishi, M.J., S.Kimura, H.Nakata and Y.Yamashita (1990}: A biomass-based model for the sand lance (Ammodytes personatus) in Seto Inland-sea (Japan). Ecol. Modelling (in press). Nakata, H.(1988): Wind effects on the transport of Japanese sand eel larvae in the eastern part of the Seto Inland Sea.
Nippon Suis~In Cakkaishi,
54, 1553-1561.
199