Diurnal migration and vertical distribution of ... - Springer Link

Marine Biology 54, 109-124 (1979)

MARINE BIOLOGY 9 by Springer-Verlag 1979

Diurnal Migration andVertical Distributionof Phyllosoma Larvae of the Western Rock Lobster Panulirus cygnus* D.W. Rimmer and B,F. Phillips

Abstract

Studies

off the w e s t c o a s t of A u s t r a l i a showed that the p h y l l o s o m a larvae of P a n u l i G e o r g e u n d e r g o a d i u r n a l v e r t i c a l m i g r a t i o n , w i t h light as an i m p o r t a n t factor i n f l u e n c i n g the d e p t h d i s t r i b u t i o n of all 9 p h y l l o s o m a stages. The early stages (I to III) o c c u r r e d at the s u r f a c e at n i g h t r e g a r d l e s s of m o o n l i g h t intensity, w h e r e a s late stages (VI to IX) c o n c e n t r a t e d at the s u r f a c e only on n i g h t s w i t h less than 5% of full m o o n l i g h t . M i d d a y p e a k d e n s i t i e s of e a r l y - s t a g e l a r v a e o c c u r r e d in the 30 to 60 m d e p t h r a n g e w h i l e those of m i d (IV to VI) and late stages w e r e in the 50 to 120 m range. D e p t h s of p e a k d e n s i t i e s of l a r v a e i n c r e a s e d w i t h d i s t a n c e o f f s h o r e . The l i m i t s of v e r t i c a l d i s t r i b u t i o n of the p h y l l o s o m a rem a i n e d w i t h i n r a n g e s of i l l u m i n a n c e w h i c h w e r e e s t i m a t e d to be in the order of 50 to 250 ~E m - 2 s e c -1 for e a r l y stages, 20 to 200 ~uE m - 2 s e c -I for m i d stages and 5 to 50 ~E m - 2 s e c -I for late stages. M i n i m a l r a t e s of net v e r t i c a l m o v e m e n t w e r e estim a t e d for the larvae. E a r l y stages e x h i b i t e d m e a n net r a t e s of a s c e n t and d e s c e n t of 13.7 and 13.0 m h -I , r e s p e c t i v e l y , w h i l e the rates for mid stages were 16.O and 16.6 m h -I and for late stages 19.4 and 20.1 m h -I. D i u r n a l m i g r a t i o n s and v e r t i c a l d i s t r i b u t i o n are shown to have a v i t a l role in the r e l a t i o n s h i p s b e t w e e n c i r c u l a tion in the s o u t h - e a s t e r n I n d i a n O c e a n and the t r a n s p o r t and d i s p e r s a l of the phyll o s o m a larvae. The d i u r n a l m i g r a t i o n s of e a r l y stages place t h e m at the s u r f a c e at night, w h e n o f f s h o r e v e c t o r s of w i n d - d r i v e n o c e a n - s u r f a c e t r a n s p o r t d o m i n a t e , and b e l o w the d e p t h of w i n d - i n d u c e d t r a n s p o r t d u r i n g the day, w h e n o f f s h o r e v e c t o r s are small or n e g a t i v e , thus a c c o u n t i n g for t h e i r o f f s h o r e d i s p l a c e m e n t . M i d and late stages, b e c a u s e of their d e e p e r d a y t i m e d i s t r i b u t i o n and a b s e n c e from the s u r f a c e on m o o n l i g h t nights, are p r e d o m i n a n t l y s u b j e c t to c i r c u l a t i o n f e a t u r e s u n d e r l y i n g the i m m e d i a t e s u r f a c e layer. This is h y p o t h e s i z e d to a c c o u n t for the r e t u r n of the p h y l l o s o m a to areas near the c o n t i n e n t a l shelf edge by s u b j e c t i n g them to a c o a s t w a r d m a s s t r a n s p o r t of w a t e r w h i c h u n d e r l i e s the i m m e d i a t e s u r f a c e layer. rus cygnus

I ntroduction

light i n t e n s i t i e s in the l a b o r a t o r y have b e e n d e s c r i b e d and r e l a t e d to v e r t i c a l m i g r a t i o n by Ritz (1972a, b). V a r i o u s a u t h o r s have d o c u m e n t e d f r a g m e n tary i n f o r m a t i o n on v e r t i c a l d i s t r i b u However, t h e r e is l i t t l e d e t a i l e d or tion of the p h y l l o s o m a of s e v e r a l p a l i c o m p r e h e n s i v e i n f o r m a t i o n a v a i l a b l e on n u r i d species, e.g. P a n u l i r u s v u l g a r i s the v e r t i c a l d i s t r i b u t i o n and d i u r n a l (Russell, 1925, 1927) , P a n u l i r u s j a p o n i c u s m i g r a t i o n s of l a r v a e of any of the pal(Harada, 1957; Saisho, 1966), p a n u l i r u s inurid species. As r e c e n t l y as 1967 argus (Smith, 1958; Sims and Ingle, 1966). t h e r e was some d o u b t e x p r e s s e d as to Panulirus interruptus (Johnson, 1960, 1971), w h e t h e r p a l i n u r i d p h y l l o s o m a a c t u a l l y P a n u l i r u s cygnus ( C h i t t l e b o r o u g h and u n d e r g o v e r t i c a l m i g r a t i o n at all (LazaThomas, 1 969) , P a n u l i r u s spp. (Austin, rus, 1967). 1972). In a d d i t i o n , some r e s p o n s e s of The w e s t e r n r o c k l o b s t e r p a n u l i r u s cygp h y l l o s o m a of P. cygnus to l o w - l e v e l n u s is c o n f i n e d to the w e s t c o a s t of, A u s t r a l i a b e t w e e n a p p r o x i m a t e l y 22o00 ' *The western rock lobster is referred to as P. and 34~ The d i s t r i b u t i o n of the l o n g i p e s or P. l o n g i p e s c y g n u s in some of the a d u l t stock is c o n c e n t r a t e d b e t w e e n latliterature quoted; these are synonymous with itudes 27000 ' and 3 2 ~ (ChittleboP. cygnus. r o u g h and Thomas, 1969).

0025-3162/79/0054/0109/S03.20

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D.W. Rimmer and B.F. Phillips: Diurnal Migration of P a n u l i r u s Phyllosoma

P r e s e n t k n o w l e d g e i n d i c a t e s t h a t circ u l a t i o n of the s o u t h - e a s t e r n I n d i a n O c e a n is c o m p l e x and c a n n o t be d e s c r i b e d as c h a r a c t e r i z e d or d o m i n a t e d by any single f e a t u r e (see Wyrtki, 1962, 1973; Hamon, 1965, 1972; Rochford, 1969, 1977; W y r t k i et ai.,1971; Andrews, 1977; Kitani, 1977). C u r r e n t s for the m o s t part are v a r i a b l e and weak. M a s s t r a n s p o r t of waters in the upper 300 m is g e n e r a l l y towards the west c o a s t of A u s t r a l i a t h r o u g h o u t the year (Wyrtki et al., 1971; Wyrtki, 1973), but r a n d o m v a r i a t i o n in g e o s t r o p h i c c u r r e n t s of a n o n - s e a s o n a l n a t u r e f r e q u e n t l y c o m p l i c a t e or o b s c u r e the g e n e r a l p i c t u r e (e.g. Hamon, 1972). A large c y c l o n i c eddy or gyre of v a r i able strength, a p p r o x i m a t e l y 100 km in diameter, c e n t r e d a b o u t 32~ 1 1 o O 0 0 ' E and w i t h an o n s h o r e f l o w in the r e g i o n of 29o00 ' to 31~ was des c r i b e d as a p e r m a n e n t f e a t u r e of the c i r c u l a t i o n by W y r t k i (1962). A n d r e w s (1977) d e s c r i b e d this f e a t u r e as a m e a n dering current associated with a number of " m e s o - s c a l e " e d d i e s about 100 to 2 O O k m i n d i a m e t e r w h i c h p r o d u c e d a coastward f l o w in the same region, then t u r n e d s o u t h w a r d to run p a r a l l e l to the c o n t i n e n t a l shelf. E d d i e s of a s i m i l a r n a t u r e w e r e d e s c r i b e d in the area by C r e s s w e l l (1977), K i t a n i (1977), and G o l d i n g and S y m o n d s (1978). A p p l i c a t i o n of the c l a s s i c a l t h e o r y of w i n d - d r i v e n c i r c u l a t i o n off the w e s t c o a s t of Aust r a l i a showed s u r f a c e - t r a n s p o r t v e c t o r s w h i c h w e r e s e a s o n a l l y v a r i a b l e and did not n e c e s s a r i l y c o i n c i d e w i t h the m o v e m e n t of u n d e r l y i n g w a t e r (Cresswell, 1 972). S t u d i e s of the larvae of P a n u l i r u s cygn u s by C h i t t l e b o r o u g h and T h o m a s (1969), P h i l l i p s et al. (I 978), P h i l l i p s et al. (in preparation) and R i m m e r (unpublished data) have shown that h a t c h i n g takes p l a c e from N o v e m b e r to F e b r u a r y and that the p h y l l o s o m a are t r a n s p o r t e d o f f s h o r e into the Indian Ocean, p r o b a b l y by windd r i v e n s u r f a c e currents. T h e m e t h o d of r e t u r n of the p h y l l o s o m a is less well u n d e r s t o o d , but r e s u l t s in a c o n c e n t r a tion of l a t e - s t a g e l a r v a e in the w a t e r s off the c o n t i n e n t a l shelf of the w e s t coast of A u s t r a l i a b e t w e e n A u g u s t and December. F r o m this p o i n t the larval p h a s e is c o m p l e t e d by the p u e r u l u s stage, w h i c h m e t a m o r p h o s e s from the last phyllosoma stage and then a c t i v e l y swims across the c o n t i n e n t a l shelf w a t e r s to settle in the s h a l l o w i n s h o r e reef areas, This paper p r e s e n t s the r e s u l t s of studies of the d i u r n a l m i g r a t i o n s and v e r t i c a l d i s t r i b u t i o n s of the p h y l l o s o m a larvae, c a r r i e d out d u r i n g the years 1974 to 1977 as part of an e x t e n s i v e study of the larval e c o l o g y of the w e s t -

ern r o c k lobster. R e s u l t s are d i s c u s s e d in r e l a t i o n to the c i r c u l a t i o n of the s o u t h - e a s t e r n Indian Ocean, w i t h the imp l i c a t i o n s of d i u r n a l m i g r a t i o n and v e r tical d i s t r i b u t i o n in the t r a n s p o r t and d i s p e r s a l of the larvae.

Materials and Methods

Seven c r u i s e s w e r e d e s i g n e d to p r o v i d e d i u r n a l series of s a m p l e s from s e l e c t e d l o c a t i o n s for d i f f e r e n t stages of d e v e l o p m e n t of the p h y l l o s o m a l a r v a e of p a n u l i r u s c y g n u s George. A f u r t h e r 14 c r u i s e s w e r e d e s i g n e d to s a m p l e r e p e t i t i v e l y either e a r l y - or l a t e - s t a g e p h y l l o s o m a of p. c y g n u s at night, at fixed s t a t i o n s along a series of t r a n s e c t s in 3 areas on a f o r t n i g h t l y or m o n t h l y basis. L o c a tions of the s a m p l i n g s t a t i o n s are shown in Fig. I. The s a m p l i n g p r o c e d u r e at e a c h station was b a s i c a l l y the same for all cruises. A h o r i z o n t a l haul was t a k e n w i t h p a i r e d nets at the s u r f a c e and 4 s u b s u r f a c e d e p t h s to c o n c u r r e n t l y o b t a i n q u a n t i t a t i v e s a m p l e s at each of the s e l e c t e d d e p t h strata. The d e p t h s sampled w e r e over a s h a l l o w e r r a n g e d ~ r i n g the n i g h t than in the day, so that the limits of larval d e p t h d i s t r i b u t i o n w o u l d be e n c o m p a s s e d w i t h a m i n i m a l d e p t h i n t e r v a l b e t w e e n nets. The d e p t h r a n g e s s a m p l e d w e r e v a r i e d for the diff e r e n t p h y l l o s o m a stages of P a n u l i r u s c y g n u s being sought, and were also a d j u s t e d as e x p e r i e n c e led to m o r e e f f e c t i v e d e p t h s p a c i n g of the samplers. At night, the m a x i m u m d e p t h r a n g e s s a m p l e d were O to 35 m on the c o n t i n e n t a l shelf and 0 to 90 m off the shelf. D u r i n g the day, d e p t h s from O to a m a x i m u m of 1OO to 300 m w e r e sampled. The s a m p l e r s and t e c h n i q u e s used have been d e s c r i b e d p r e v i o u s l y (Phillips and Rimmer, 1975; G r i f f i t h s and Rimmer, 1977). The d e s i g n of the s u b s u r f a c e s a m p l e r s was such that they p r e s e n t e d a pair of u n o b s t r u c t e d net m o u t h s w h e n under t o w and f i l t e r e d a negligible volume during deployment and r e c o v e r y . S u r f a c e nets w e r e t o w e d from booms, one on each side of the ship. E a r l y - s t a g e p h y l l o s o m a l a r v a e w e r e collected w i t h 1.O5 m m a p e r t u r e mesh, 0.25 m 2 m o u t h nets, t o w e d for 30 min at 1.O m sec -I (2 knots) and later stages w e r e c o l l e c t e d w i t h 2.5 m m a p e r t u r e mesh, 0.25 m 2 m o u t h nets, towed for 60 to 75 m i n at 2.5 m sec -I (5 knots). E a c h s u r f a c e sampler was fitted w i t h a flowm e t e r and e a c h s u b s u r f a c e sampler w i t h a depth/distance recorder. In a d d i t i o n to the p l a n k t o n samples, at each s t a t i o n a b a t h y t h e r m o g r a p h and/ or e x p e n d a b l e b a t h y t h e r m o g r a p h cast,

D.W. Rimmer and B.F. Phillips: Diurnal Migration of Panulirus Phyllosoma

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Fig. I. Study area, showing locations of stations where phyllosoma larvae were sampled. Nocturnal sampling along transects is indicated by arrows: § early stages, fortnightly, 24 November 1974 to 28 February 1975, coinciding with alternating new- and full-moon periods; + late stages, monthly, 30 July 1975 to 4 September 1975; + late stages, monthly, 15 September 1975 to 21 November 1975

surface-water temperature, surface-sal i n i t y s a m p l e and a n u m b e r of s t a n d a r d meteorological observations including sea state, swell, w i n d f o r c e and c l o u d c o v e r w e r e taken. M o o n l i g h t i l l u m i n a n c e at the s u r f a c e was c o m p u t e d for each n i g h t s t a t i o n u s i n g the m e t h o d of A u s t i n et al. (1976). C o r r e c t i o n s for m o o n l i g h t r e d u c t i o n by c l o u d c o v e r w e r e i n c o r p o rated. D a y l i g h t i r r a d i a n c e was m e a s u r e d at 5 to 10 m d e p t h i n t e r v a l s t h r o u g h the u p p e r 1OO m of the w a t e r c o l u m n at 32o00'S, 1 1 5 ~ on 25 April, 1978. R e a d i n g s w e r e t a k e n at a p p r o x i m a t e l y h o u r l y i n t e r v a l s over the d a y l i g h t p e r i od w i t h a s u b m a r i n e q u a n t u m m e t e r (Lambda L I - 1 8 5 w i t h L I - 1 9 2 S u n d e r w a t e r q u a n t u m sensor). T h e p h y l l o s o m a of Panulirus cygnus w e r e sorted and d e s i g n a t e d S t a g e s I to IX u s i n g the key of B r a i n e et al. (1979). In m a n y i n s t a n c e s , e i t h e r the n u m b e r s of l a r v a e of p a r t i c u l a r s t a g e s w e r e too low to p e r m i t i n d i v i d u a l a n a l y s i s , or similarities between stages made separate t r e a t m e n t r e p e t i t i o u s . To e x p e d i t e t r e a t ment, the l a r v a e w e r e a r b i t r a r i l y g r o u p e d into e a r l y stages (I to III),

m i d stages (IV to VI) and late stages (VI to IX). A p r o b l e m a r o s e w i t h S t a g e VI, b e c a u s e it c o m p r i s e d s e v e r a l m o r phologically indistinguishable instars ( p r o b a b l y 5), w h i l e t h e o t h e r s t a g e s each c o m p r i s e d o n l y I or 2 instars. T h e s m a l l e r i n s t a r s of Stage VI w e r e f o u n d e a r l i e r in the year (January to May), and u s u a l l y d i s t r i b u t e d g e o g r a p h i c a l l y w i t h the m i d stages, w h i l e the l a r g e r i n s t a r s a p p e a r e d later (May to November) and w e r e a s s o c i a t e d m o s t f r e q u e n t l y w i t h late s t a g e s (Phillips et al., in p r e p a r a tion). Stage VI w e r e a s s i g n e d to e i t h e r the m i d - or l a t e - s t a g e g r o u p a f t e r cons i d e r a t i o n s of size, t i m e of year and the c o n t e x t of the sampling. N u m b e r s of p h y l l o s o m a in the c a t c h e s w e r e c o n v e r t e d to d e n s i t i e s for each s t r a t u m sampled, and e x p r e s s e d as number per 1000 m 3 of w a t e r filtered. E s t i m a t e s of n u m b e r s of l a r v a e in the w a t e r c o l u m n u n d e r a u n i t of s u r f a c e area w e r e c a l c u l a t e d by m u l t i p l y i n g the larval d e n s i t y at each d e p t h s a m p l e d by the n u m b e r of m e t r e s in e a c h d e p t h s t r a t u m s a m p l e d in the haul and s u m m i n g them. E s t i m a t e d n u m b e r s of l a r v a e in the w a t e r

D.W. Rimmer and B.F. Phillips: Diurnal Migration of Panulirus Phyllosoma

112

near the surface. The onset of the m o r n ing d e s c e n t was not c l e a r l y d e f i n e d because the s a m p l e s o b t a i n e d in the d a w n p e r i o d s of both series had low catches. By 0 8 . 0 0 to 0 9 . 0 0 hrs, the peak d e n s i t y was m o v i n g into the 30 to 60 m d e p t h r a n g e and by 10.00 hrs the p e a k d e n s i t y was e s t a b l i s h e d in that r a n g e w h e r e it r e m a i n e d t h r o u g h the m i d d a y period. V e r y few early stages w e r e ever t a k e n in s a m p l e s at d e p t h s g r e a t e r than 60 m. The data p r e s e n t e d a b o v e on earlystage v e r t i c a l d i s t r i b u t i o n at n i g h t w e r e c o m p a r e d w i t h d a t a from an addit i o n a l 131 p l a n k t o n hauls t a k e n at n i g h t (Rimmer, u n p u b l i s h e d data) d u r i n g surveys of e a r l y - s t a g e p h y l l o s o m a in 1974 and 1975. S i m i l a r i t y b e t w e e n v e r t i c a l d i s t r i b u t i o n s of the p h y l l o s o m a from the two sets of o b s e r v a t i o n s , w h e r e e n v i r o n mental conditions were comparable, s t r o n g l y s u p p o r t e d the r e s u l t s d e s c r i b e d above.

c o l u m n for day and n i g h t hauls w i t h i n each cruise w e r e c o m p a r e d u s i n g W i l c o x o n ' s rank sum test. T h e r e was no s i g n i f i cant d i f f e r e n c e (P