A shallow-water fish assemblage, over a soft, sandy substratum at Porto Pim. Bay on the island of Faial, Azores, was sampled at monthly intervals with a.
Estuarine, Coastal and Shelf Science (1995) 41, 579-591
Seasonal Changes in a Sandy B e a c h Fish A s s e m b l a g e at Porto P i m , Faial,-Azores
Ricardo S. Santos a and Richard D. M. Nash b ~Departamento de Oceanografia e Pescas, Universidade dos A~ores, PT-9900 Horta, Faial, Azores, Portugal and bPort Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA, U.K. Received 19 August 1993 and in revised form 13 September 1994
Keywords: Azores; shallow water; fish; seasonal A shallow-water fish assemblage, over a soft, sandy substratum at Porto Pim Bay on the island of Faial, Azores, was sampled at monthly intervals with a beach seine. Sampling was undertaken at 3-h intervals over 24 h'on a monthly basis between July 1989 and June 1990. There was seasonal variation in the density of individuals and biomass with the highest values in summer (1 "4 m - 2 and 27-75 g m 2 respectively). A total of 24 species were observed and the assemblage was dominated by four species (Chelon labrosus, Sardina pilchardus, Pagellus bogaraveo, Trachinotus ovatus). Species diversity (H') was high throughout the year but equatability (_7') was low, due to the dominance of a few species. The largest change in the assemblage structure occurred between March and April. Only a few species could be classified as resident, with the two predators (Dasyatis pastinaca and Echiichthys vipera) being abundant at different times of the year. Most of the species observed were juvenile fish, many with an offshore distribution as adults, using Porto Pim Bay as a nursery ground. c~ 1995 Academic Press Limited
Introduction T h e r e have b e e n a n u m b e r of studies which have e x a m i n e d the seasonal variations in shallow-water fish assemblage structure o n soft sediments ( H o r n , 1980; Allen & D e M a r t i n i , 1983; Allen et al., 1983; Lasiak, 1984b; N a s h , 1988; Wright, 1988, 1989; Ali & H u s s a i n , 1990). T h e s e studies vary from descriptions of the fish assemblages a n d their d y n a m i c s (e.g. H o r n , 1980) to a n e x a m i n a t i o n of the effects of severe seasonal changes in e n v i r o n m e n t a l c o n d i t i o n s (Mash, 1988). Shallow-water, s a n d - b o t t o m areas in the Azores are relatively scarce a n d it is only recently that this habitat has b e e n studied (Nash et al., 1990, 1991, 1994a; N a s h & Santos, 1993). T h e r e is a fairly large range in the potential density of fish a n d s t a n d i n g crops in shallow-water areas (Ross et al., 1987), a n d it is u n k n o w n whether the relatively rare shallow-water habitat in the Azores is high or low with respect to these other temperate a n d subtropical areas. T h e Azores islands are also generally well separated a n d as a group they are approximately 1700 k m from the nearest c o n t i n e n t . D u e to the close proximity of very deep water, there is also the possibility of interactions b e t w e e n coastal a n d oceanic species as well as shallow-water a n d deep-water species. 0272-7714/95/050579+ 13 $12.00/0
© 1995 Academic Press Limited
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R. S. Santos & R. D. M. Nash
This study provides a description of the seasonal fluctuations in a shallow-water, soft-sediment fish assemblage, and c o m p a r e s this with o t h e r studies u n d e r t a k e n elsewhere b o t h in other subtropical and in t e m p e r a t e areas.
S t u d y site T h e Azores are a small group of islands (which cover a fairly large area) located along the m i d - A t l a n t i c ridge in the region of 38°N 28°W. T h e s a m p l i n g site, Porto Pim, is located on the south-east corner of the island of Faial (Figure 1). T h e beach is small (approximately 280 m in length) facing south-west. T h e n o r t h a n d south coasts of the beach area are rocky. T h e southern b o r d e r is the edge of the M o n t e da G u i a p r o t e c t e d area. T h e m a x i m u m tidal range here is 1 m. T h e m a x i m u m horizontal intertidal area of the beach at low water is a p p r o x i m a t e l y 30 m.
Methods Sampling was u n d e r t a k e n with a 20 m b e a c h seine (32 m m stretch m e s h in the wings reducing t h r o u g h 15 m m to 8 m m in the centre). T w o sets of the b e a c h seine were m a d e at 3-h intervals for 24 h on each s a m p l i n g date. S a m p l i n g c o m m e n c e d in July 1989 a n d finished in June 1990 at a p p r o x i m a t e l y 1 - m o n t h intervals. E x t r e m e l y b a d w e a t h e r in D e c e m b e r 1989 p r e v e n t e d s a m p l i n g on that occasion. T h e b e a c h seine was fitted with 25 m hauling ropes and set parallel to the beach. T h e net swept an area from a p p r o x i m a t e l y 4 m d e p t h to the shore, with each haul having an effective fishing area o f 291 m 2 [(using the m e t h o d o l o g y of K u b e c k a & B o h m (1991)]. N o estimates o f overall efficiency are available for this net on this beach. W a t e r t e m p e r a t u r e was o b t a i n e d from H o r t a H a r b o u r as part o f the general daily m o n i t o r i n g p r o g r a m m e c o n d u c t e d by the D e p a r t m e n t of O c e a n o g r a p h y a n d Fisheries, University of the Azores, Horta. T h e data given here are for the specific s a m p l i n g dates used. All species were identified using W h i t e h e a d et al. (1986). T h e total n u m b e r of individuals and total weight for each species in each haul was obtained. T h e lengths (to the nearest 1 m m ) a n d individual weights (to the nearest 1 g) (using a s u b s a m p l e of up to 100 individuals where necessary) were m e a s u r e d for each sample. In this report, all m o n t h l y catches are r e p o r t e d as a s u m m a t i o n of the two hauls taken at each s a m p l i n g time and the eight samples taken over the 24-h p e r i o d (i.e. 16 samples). Average catch (numbers and biomass) was calculated as the g e o m e t r i c m e a n (log~o n + 1). Species diversity was m e a s u r e d from the n u m b e r of species a n d the S h a n n o n diversity index (H') (Pielou, 1977). the equatability b e t w e e n species in the c o m m u n i t y was m e a s u r e d with Pielou's evenness function (J') (Pielou, 1977). All calculations utilized logarithms to base 10. T h e c o m m u n i t y p a r a m e t e r s were calculated for each m o n t h . T h e overall assemblage p a r a m e t e r s for the whole year were calculated from the m e a n catch for the whole sampling p e r i o d (see N a s h , 1988). T h e percentage similarity b e t w e e n successive s a m p l i n g data was calculated using the m e t h o d of W h i t t a k e r and F a i r b a n k s (1958) b a s e d on species a b u n d a n c e data.
Azorean shallow-waterfish
581
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Results Sea-surface temperature T h e o n l y p h y s i c a l v a r i a b l e to b e m e a s u r e d was s e a - s u r f a c e t e m p e r a t u r e . T h e r e was a s e a s o n a l cycle w i t h h i g h t e m p e r a t u r e s (21.9 °C) in J u l y w h i c h d e c l i n e d to a m i n i m u m (14-7 °C) in M a r c h ( F i g u r e 2).
R. S. Santos & R. D. M. Nash
582
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Month Month Figure 2. Seasonal variations in the community parameters of a shallow water fish assemblage in Porto Pim Bay, Faial, Azores between July 1989 and June 1990. (a) Numbers of species, (b) numbers of individuals caught (solid line) and temperature (dashed line), (c) biomass caught (solid line) and temperature (dashed line), (d) species diversity (/4') (solid line) and evenness (J') (dashed line).
Fish assemblage T w e n t y - f o u r species were caught yielding 39 350 fish or 619.9 kg. T h i s constitutes a yearly average of 223 fish 1 0 0 0 m 2 or 4-5 kg 1 0 0 0 m 2 ( T a b l e 1). Over the four seasons, estimated density was highest in s u m m e r (1.4 individuals m 2) [Figure 3(a)] and lowest in winter. T h e highest b i o m a s s also o c c u r r e d in s u m m e r (27'75 g m 2) followed by a u t u m n , spring and then winter [Figure 3(b)]. T h e d o m i n a n t species with respect to n u m b e r s of individuals a n d b i o m a s s was Chelon labrosus (64 a n d 89% respectively) ( T a b l e 1). T w o species (C. labrosus, Trachinotus ovatus) constituted the majority of the assemblage with respect to n u m b e r s (91%) a n d biomass (97%). O f the species caught, only three species could be c o n s i d e r e d residents of soft-sediment areas and Porto P i m Bay in particular (Dasyatis pastinaca, Echiichthys vipera and Bothus podas). However, there is a certain degree o f d o u b t as to w h e t h e r these species are true residents as a b u n d a n c e s c h a n g e d for D. pastinaca a n d E. vipera suggesting a possible m o v e m e n t out o f the area, a n d the majority o f B. podas were juveniles which p r o b a b l y move off this nursery g r o u n d to the s u r r o u n d i n g rocky areas.
Number of species T h e r e did not a p p e a r to be any clear p a t t e r n in the n u m b e r of species caught at P o r t o P i m [Figure 2(a)]. T h e m a x i m u m n u m b e r o c c u r r e d in S e p t e m b e r (16 species). I n general, there were 9-11 species present each m o n t h .
Azorean shallow-water fish
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TABLE 1. Geometric m e a n abundance and biomass offish caught with a beach seine at Porto Pim Bay over the period July 1989 to June 1990 (no samples in December 1989)
Average number Scientific name
Common name
Dasyatis pastinaca Synodus saurus Sardina pilchardus Belone belone Capros aper Pomatomus saltator Trachurus picturatus Trachitzotus ovatus Pseudocaralzx dentex Mulhts sttrT~ltllettts Pagellus bogaraveo Boops boops Diplodus sargus Chelon labrosus Sarpa salpa Echiichthys vipera Bothus podas Sphoeroides mannoratus Syngnathus acus Conger conger Pagellus acarste Myctophid Unidentified
C o m m o n stingray
Atlantic lizard fish Pilchard Garfish Boarfish Bluefish Blue jack mackerel Pompano Guelly jack Striped red mullet Red sea bream Bogue White sea bream Thicldip grey mullet Salema Lesser weever
Wide-eyed flounder Guinean puffer Greater pipefish Conger eel Auxiliary sea bream Myctophid
Number of species To tal nos/wt
(1000 m - 2 ) 0.29 0.01 6-04 0"07 0'04 0.43 0.74 59.18 0.07 1.15 5.19 0-26 2.29 142-35 0.13 1.06 2.25 0.52 0.01 0.01 0' 16 0.03 0.01 24 222.59
/~verage weight % 0.13 0-01 2.71 0"03 0.02 0"33 0.33 26.59 0.03 0-52 2.33 0.12 1 "03 63.95 0-06 0.47 1.01 0-23 0-01 0.01 0.07 0.01 0.01
(g 1 0 0 0 m -2) 4.81 0-08 43.84 0" 10 0.14 5.63 16.89 349.45 0"35 18.14 10-30 1.75 21.78 3966.65 1.42 4-84 5.81 3-07 + • 0-18 0.31 0.05 0-04
% 0" 11 + 0"98 + + 0.13 0.38 7.84 + 0.41 0.23 0.04 0.49 89-03 0.03 0.11 0" 13 0.07 + + + + +
24 4455.65
+, value
