BULLETIN OF MARINE SCIENCE, 72(3): 675–683, 2003

DIET COMPOSITION AND FEEDING HABITS OF , CHRYSURUS (PISCES: ), LARVAE IN THE SOUTHERN GULF OF MEXICO

Marina Sánchez-Ramírez

ABSTRACT Chloroscombrus chrysurus (Atlantic bumper) is an ecologically important in the southern Gulf of Mexico, as it is abundant throughout the area and contributes to the diet of a great number of species of recreational and commercial value. For this reason, changes in the size of its populations can have an effect on these . The larval stage of generally has the highest mortality rate, and because mortality rate is often influenced by prey availability, this study focused on the feeding habits of Atlantic bumper larvae. samples were collected during five oceanographic cruises (1987– 1993) and the digestive tracts of 297 larvae, 1.03–8.07 mm SL, were analyzed. Larvae of this species fed mainly during the day, and feeding incidence was greatest for postflexion larvae (values of 100%) when the larvae capture their prey more efficiently. Larvae fed primarily on Penilia avirostris (Cladocera), copepods and nauplii, and the number and size of prey increased with larval development.

Fish play a very important part in the dynamics of marine ecosystems. Year class strengths are largely determined during the larval stage because variability in the high mortality rates during the early life history stages may result in large differences in the magnitude of recruitment and abundance of the adult population (O’Connell and Raymond, 1970; Cushing, 1975; Laurence, 1977; Ivlev, 1961 vide in Last, 1978; Houde, 1978; Hunter, 1981; Lasker, 1981; Hjort, 1914, 1926 vide in Towsend, 1983; Hjort, 1914 vide in Hempel, 1984). The survival of larvae depends upon such factors as transportation by currents, ad- equate food and the evasion of predators (Ahlstrom and Moser, 1976). The abundance of food directly affects growth and mortality rates (Ivlev 1961 vide in Last, 1978; Waytt, 1972). Well-fed organisms are more robust, active, and less subject to and dis- eases. Their ability to search for food also increases (May, 1974; Laurence, 1977; Last, 1978; Alvariño, 1985). McGurk (1986, 1987) proposed that temperature controls the rate of development of larvae, and may control the duration of the period within the interaction between preda- tion mortality and patchiness operates, but Pepin (1991) also pointed out that high growth rates require high ingestion rates, which in turn require that greater numbers of prey items be encountered and results in increased encounters with predators. Because feeding is important to mortality of the early life stages of fishes, information on the diet of larvae is necessary in population studies. The Atlantic bumper (Chloroscombrus chrysurus) is a typical and ecologically domi- nant species in the fish community of the southern Gulf of Mexico (Yáñez-Arancibia and Sánchez-Gil, 1986). It spawns inside the 40 m isobath throughout the year, with a maxi- mum during the warm spring–summer season (Flores-Coto and Sánchez-Ramírez, 1989). The Atlantic bumper constitutes an important food source for many fish of recreational and commercial value, thus the abundance of Atlantic bumper may ultimately affect the biomass of other fishes (Shaw and Drulliger, 1990).

675 676 BULLETIN OF MARINE SCIENCE, VOL. 72, NO. 3, 2003

The purpose of this study was to determine the diet and feeding habits of Atlantic bumper larvae during development and among seasons, in the southern Gulf of Mexico.

METHODS

FIELD METHODS.—The study area was located in the southern Gulf of Mexico, at 18°06'– 21°00' N and 90°26'–97°20' W (Fig. 1). Zooplankton samples were collected on board the R/V JUSTO SIERRA during five cruises between 1987–1993 (Table 1). Zooplankton sampling consisted of a double oblique plankton tow following a circular trajectory using a Bongo net with 333 and 505 µm mesh sizes. During the Mopeed V cruise, sampling was carried out at several depths (2, 10, 20 and 45 m) throughout the water column, with an opening-closing net with a 75 cm diameter mouth and a 505 µm mesh size netting. Samples were fixed with 5% formalin neutralized with sodium borate. LABORATORY ANALYSES.—The Atlantic bumper larvae were separated, identified, and preserved in 70% ethanol. Larvae that were not twisted and curled were selected for the diet analysis. They were measured for standard length (SL) or notochordal length when the flexion of the notochord was not complete, and for the length of the upper jaw, under a dissecting microscope with a micrometric ocular and a 0.01 mm precision. The C. chrysurus larvae were grouped by stage of development into preflexion (3.0 mm SL), flexion (3.1–4.0 mm SL) and postflexion (4.1–9.0 mm SL), as established by Sánchez- Ramírez and Flores-Coto (1993). The digestive tracts of 297, 1.30–8.07 mm SL, Atlantic bumper larvae collected during the four climatic seasons were dissected in a damp cham- ber using very fine needles. The contents were identified to the lowest possible taxon and counted. The prey were measured for width (short axis) under a microscope with a micrometric ocular. DATA ANALYSES.—The feeding incidence was calculated as the percentage of larvae with food in the digestive tract with respect to the number of larvae analyzed for the day (6:40–17:40 hrs) and night (20:36–5:58 hrs) periods, as well as for each stage and cli- matic season. The index of relative importance (IRI) proposed by George and Hadley

Figure 1.- Study area. SANCHEZ-RAMIREZ: FEEDING OF ATLANTIC BUMPER LARVAE 677

Table 1. Summary of research cruises to the southern Gulf of Mexico.

Ceruise Dnat Sseaso Number of Station O7GMEX II 2r7 July– 5 August, 198 s8umme 6 O7GMEX III 2l8 November– 5 December, 198 f4al 4 O8GMEX V Arugust 1– 9, 198 s6umme 7 M2OPEED II Jgune 20– 24, 199 s1prin 2 M3OPEED V Frebruary 12– 18, 199 w0inte 2

(1979) and modified by Towsend (1983) was calculated to analyze the importance of each food category in the diet of the Atlantic bumper larvae at each developmental stage and for each climatic season:

100()Xa IRIa = n ∑ Xa a=1

where IRIa= Index of relative importance for food item a, Xa = % frequency of occurrence + % number of food item a, and n = number of different food items in the larvae of each sample. The size of the mouth was calculated using the index proposed by Shirota (1970) to analyze the relationship between the size of the mouth and the size of the injested prey:

MUJ= 2()

where M = relative mouth size (mm) and UJ = length of the upper jaw (mm).

RESULTS AND DISCUSSION

FEEDING INCIDENCE.—Chloroscombrus chrysurus demonstrated a clear tendency to feed during the day. Of the 181 larvae collected during the day (6:40–17:40 hrs) feeding inci- dence was 79.6%, whereas of the 116 collected during the night (20:36–5:58 hrs) feeding incidence was only 12.9% (Table 2). As Hunter (1981) mentioned, marine fish larvae are visual feeders. The feeding is confined to daylight hours as indicated by analysis of other species (Arthur, 1976). In nearly all species of fish the visual cells comprise only cones in the larval stage and there is no movement of retinal masking pigment when the larvae are subjected to changing conditions of illumination (Blaxter and Staines, 1970). A pure- cone retina is thus adequate for first feeding, and experiments have demonstrated that light is required for feeding by many species, at least in the early larval stages (Blaxter,

Table 2. Diel feeding incidence of Chloroscombrus chrysurus larvae collected in the southern Gulf of Mexico. N = number of digestive tracts analyzed.

HNdR Wdith foo Wdithout foo % with foo 6:40−117:40 148 174 3679.5 20:36−065:58 151 11130 12.9 678 BULLETIN OF MARINE SCIENCE, VOL. 72, NO. 3, 2003

1986) when they need light to locate their prey, which they do mainly by sight. This has been observed in the majority of fish larvae (Hunter, 1981) including clupeids such as Brevoortia tyrannus (June and Carlson, 1971), B. patronus (Govoni et al., 1983) and Sardinops sagax (Arthur, 1976), engraulids such as Engraulis mordax (Arthur, 1976), scombrids such as Katsuwonus pelamis, Thunnus maccoyi and T. alalunga (Young and Davis, 1990), carangids such as Trachurus declivis (Young and Davis, 1992) and sciaenids such as Leiostomus xanthurus and Micropogonias undulatus (Govoni et al., 1983), among others. The digestive tracts of Atlantic bumper larvae smaller than 3.0 mm SL contained food, in contrast with another carangid, Trachurus symmetricus, whose larvae must be > 3.0 mm SL to start feeding (Arthur, 1976, 1977). Although these are not closely related spe- cies, it is possible that this difference in size at the start of feeding is characteristic of organisms of lower latitudes being able to develop faster (Sánchez-Ramírez and Flores- Coto, 1993), and capture prey more efficiently at smaller sizes. Size at the time of hatch- ing differs between the two species as well: 0.76–1.02 mm SL for Atlantic bumper (Sánchez-Ramírez and Flores-Coto, 1998); 1.9–2.4 mm SL for the Jack mackerel, T. symmetricus, (Alhstrom and Ball, 1954 vide in Watson et al., 1996), which in this case results in the larvae smaller development at 3.0 mm. On the other hand, Arthur (1976) suggested that the high feeding incidence may be related to a looped gut in the larvae of these species, which reduces regurgitation at the moment of capture and preservation. Thus, as the smaller larvae of the Atlantic bumper have a more markedly looped gut than those of the jack mackerel, regurgitation may be reduced and the presence of food more obvious in Atlantic bumper larvae. Feeding incidence increases as larvae grow, and all larvae in the postflexion stage (4.01– 8.70 mm SL) collected during the day contained food in their guts (Table 3). This stage has eyes that are totally pigmented, locomotion structures (fins) that are almost com- pletely formed, and a considerably increased probability of capturing food. Such increase in feeding incidence has been observed in some other species with looped guts such as some pleuronectiforms (Last, 1978) and in T. symmetricus; whereas, a decrease in the feeding incidence has been recorded in species with a long and straight gut such as E. mordax and S. sagax (Arthur, 1976). The feeding incidence estimated for this species during the day (79.6%) takes into consideration the entire development, and is slightly higher than that reported by Young and Davis (1992) for another species of the same , T. declivis (78%). Compara- tively, Arthur (1976) reported an incidence of 90% for 7–10 mm SL T. symmetricus, and this study found a feeding incidence as high as 95.45% for 4–8 mm SL Atlantic bumper larvae. Different geographic areas often have different hydrological characteristics and water column productivities, which affect the feeding incidence of the fish larvae (Govoni and Chester, 1990). The very obvious high feeding incidence of these species of car-

Table 3. Feeding incidence of larval Chloroscombrus chrysurus collected at daylight hours, in the southern Gulf of Mexico. Spring, summer, fall and winter. N = number of digestive tracts analyzed.

Ssize (mm SL) SNdtage Wdith foo Wdithout foo % with foo 1.30−3n.00 P2reflexio 68243645.1 3.01−4n.00 F4lexio 222291.6 7 4.01−8n.70 P2ostflexio 4240100.0 0 SANCHEZ-RAMIREZ: FEEDING OF ATLANTIC BUMPER LARVAE 679

angids indicates that they are likely voracious predators, and have a low rate of digestion and/or a low rate of regurgitation. VARIATIONS IN FEEDING.—The Atlantic bumper larvae consume mainly zooplankton (Table 4). Of the 803 prey collected from the digestive tracts of this species, only one

Table 4. Index of relative importance (IRI) of food items for larval Chloroscombrus chrysurus in the southern Gulf of Mexico. N = number of food items, FO = frequency of occurrence, n = number of larvae examined, n' = number of larvae with empty guts.

preflexion flexion postflexion Spring (1.60−2.90 mm SL) (3.04−3.96 mm SL) (4.12−6.64 mm SL) n = 28 n' = 25 n = 17 n' = 9 n = 9 n' = 5 DNFIiet Items INFIR INFIR IR C224opepods 452446. Penilia avirostris 1113160 0 542040. A116mphipods 13. O252rganic matter preflexion flexion postflexion (1.30−3.00 mm SL) (3.10−4.00 mm SL) (4.10−8.07 mm SL) Summer n = 59 n' = 38 n = 22 n' = 2 n = 44 n' = 2 NFINFIR I INFIR IR Penilia avirostris 86469. 34614783. 172 3346. N542auplii 34411. 18762. 6. C115opepods 6746. 184. 160 2934. A8mphipods 101310. Z119oeae 0. O119stracods 0. I115nvertebrate eggs 6. E115quinospira larval 6. C874rustacean remains 2 O6rganic matter preflexion flexion postflexion (1.60−2.87 mm SL) (3.02−3.82 mm SL) (4.03−6.90 mm SL) Fall n = 17 n' = 15 n = 4 n' = 2 n = 31 n' = 13 NFINFIR I INFIR IR C110opepods 12200 500. 172 1952. A110mphipods 295. 15611. N1auplii 27914. P110elecypods 24325. 5. C11rustacean remains Penilia avirostris 1776 13. I117sopods 1. preflexion flexion postflexion (1.76−2.91 mm SL) (3.09−4.00 mm SL) (4.03−7.94 mm SL) Winter n = 23 n' = 17 n = 23 n' = 5 n = 20 n' = 3 NFINFIR I INFIR IR C113opepods 383. 4719520. 162 1852. N113auplii 313. 1713217. 0 4. Peridinum s113p. 33. Penilia avirostris 74140. 1331729. A534mphipods 70. 15810. I2nvertebrated eggs 117416. 1 2. C423ypris 5. Z112oeae 2. C288rustacean remains 680 BULLETIN OF MARINE SCIENCE, VOL. 72, NO. 3, 2003

Figure 2. Mouth size (Index of Shirota, 1970)–standard length (SL) relationship of larval Chloroscombrus chrysurus in the southern Gulf of Mexico.

dinoflagellate of the Peridinium was found in a preflexion larva collected in the winter, and this could have been injested accidentally. These larvae feed mainly on cope- pods, Penilia avirostris (Cladocera) and crustacean nauplii. A diet consisting largely of copepods has been reported for other carangids such as T. symmetricus (Arthur, 1976, 1977) and T. declivis (Young and Davis, 1992), and for sciaenids, such as L. xanthurus, M. undulatus (Govoni et al., 1983, 1986; Ocaña-Luna and Sánchez-Ramírez, 1998), Bairdiella chrysoura, Cynoscion nebulosus (Ocaña-Luna and Sánchez-Ramírez, 1998) and C. regalis (Goshorn and Epifanio, 1991). Penilia avirostris, copepods and crustacean nauplii are the main components of the Atlantic bumper larval diet in spring and summer, with a clear preference for the first item in all the stages during these seasons (Table 4). Atlantic bumper consumed primarily copepods during the fall and winter, when the copepods presented the greatest IRI values. The crustacean nauplii had a greater IRI in the winter than in the fall, when amphipods and juvenile bivalves had greater values than P. avirostris (Table 4). Prey selection may be determined by various factors including the distribution of preda- tors and prey, the width, swimming behavior and color of the prey (Govoni et al., 1986). The Atlantic bumper’s preference for P. avirostris during the warm seasons (spring, sum- mer) may be the result of greater availability. De la Cruz (1972) recorded a high abun- dance of P. avirostris in the Banco de Campeche during the warm months, particularly summer, coinciding with the abundance of Atlantic bumper. Prey selectivity by fish lar- vae has been reported for other species such as L. xanthurus and M. undulatus (Govoni et al., 1986), the menhaden B. patronus (Stoecker and Govoni, 1984), T. declivis (Young and Davis, 1992), T. maccoyi and T. alalunga (Young and Davis, 1990). Although it is not certain that the Atlantic bumper selects its prey, its preference for P. avirostris, which it consumes even during the season of low abundance (winter) according to De la Cruz (1972), suggests selection might take place. Searching abilities increase markedly with growth since speed, capture success rates, and perceptive distances are functions of length or age (Hunter, 1981). SANCHEZ-RAMIREZ: FEEDING OF ATLANTIC BUMPER LARVAE 681

Table 5. Width of items eaten by larval Chloroscombrus chrysurus in the southern Gulf of Mexico. N = number of items, SD = standard deviation.

Food width (short axis) (µm) Smtandard length (mm) Mminimu Meaximu ANDverag S 1.3−30.0 70219 1026. 2653. 3.1−40.0 10990 1933. 124 78. 4.1−80.7 40819 1770. 661 74.

Mouth size also affects prey selectivity (Shirota, 1970). The mouth of the Atlantic bumper larvae grows linearly with respect to the increase in standard length (Fig. 2). In the preflexion stage (1.3–3.0 mm SL) the Atlantic bumper larvae feed on organisms ≤290 µm wide; whereas, larvae >3.1 mm SL consume prey up to 900 µm wide. The average width of the injested prey is clearly greater in larvae >4.0 mm SL (Table 5). This increase in the size of the prey may also be related to the greater energy needs of the fish larvae as they grow (Blaxter, 1969; Hunter, 1981). As a result of the linear relationship between mouth size and prey size (e.g. Shirota, 1970), at the start of feeding most larvae with small mouths eat only phytoplankton, protozoans and nauplii of small copepods, whereas larvae with big mouths (Thunnus spp., Katsuwonus spp. and Seriola spp.) easily consume large copepods. In this study, for example, Atlantic bumper larvae from the preflexion stage onwards ate P. avirostris. This relationship has been observed in species such as S. sagax, E. mordax, T. symmetricus (Arthur, 1976), Leiostomus xanthurus, M. undulatus (Govoni et al., 1986), T. maccoyii (Young and Davis, 1990) and T. declivis (Young and Davis, 1992). In summary, Atlantic bumper larvae were found to feed mainly during the day, and feeding incidence was greatest for postflexion larvae (values of 100%), which are vora- cious predators and have a low rate of digestion and/or regurgitation. Larvae fed prima- rily on P. avirostris (Cladocera), copepods and crustacean nauplii, and the number and size of prey increased with larval development. This information will be useful in deter- mining the role Atlantic bumper larvae play in the trophic food web of the community, which will contribute to a better evaluation of the population and of other commercially important fish populations that feed on this species.

ACKNOWLEDGEMENTS

The author is grateful to C. Flores-Coto for access to the biological material used for this re- search, to the Instituto de Ciencias del Mar y Limnología, UNAM, to the crew of the R/V JUSTO SIERRA of UNAM, to CONACyT for the grant provided for doctoral studies, and to A. Raz-Guzmán Macbeth for the translation of the original work into English.

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DATE SUBMITTED: January 15, 2001. DATE ACCEPTED: February 18, 2002.

ADDRESSES: (MSR) Lab. Ecología, Dpto. Zoología. Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional. Prolongación de Carpio y Plan de Ayala s/n Col. Plutarco Elias Calles C. P. 11340, Mexico. E-mail: .