Growth and Mortality of Larval Atlantic Bumper <I>Chloroscombrus

BULLETIN OF MARINE SCIENCE, 63(2): 295–303, 1998 GROWTH AND MORTALITY OF LARVAL ATLANTIC BUMPER CHLOROSCOMBRUS CHRYSURUS (PISCES: CARANGIDAE) IN THE SOUTHERN GULF OF MEXICO Marina Sánchez-Ramírez and César Flores-Coto ABSTRACT Seasonal age, growth and mortality rates of larval Atlantic bumper, Chloroscombrus chrysurus, were determined from larvae collected in 13 cruises in the southern Gulf of Mexico. Age was estimated from growth increments in sagittal otoliths. One and two days, respectively, were added in the linear model to the growth increments in spring- summer and winter seasons, to estimate the probable true age. Results for spring, summer and winter are: hatching size 0.76, 0.84 and 1.02 mm; growth rates 0.17, 0.17 and 0.12 mm d−1, and mortality rates 0.30, 0.16 and 0.15 d−1, respectively. Higher temperature and food availability seem to be associated with higher growth rates in spring and summer. The lower mortality rate recorded in winter could be a consequence of low temperature, low growth rate, low larval abundance, and dilution of larval patches in the water column (due to very frequent cold fronts [Nortes] in this season). The summer mortality rate was lower than in spring, possibly a consequence of an expansion of the spawning area during summer and therefore higher larval dispersion. Many commercially important fish species have been intensively studied so that they may be optimally managed. Growth, mortality, migration, food habits, length-age relationships, distribution, and abundance studies are all common topics in the scientific literature. Some of these studies have concentrated on the early life history stages, because of its importance in the survival of these stages to the future adult stock size. Studies of larvae of non-commercial species are less frequent. Although some of these species are ecologically important and some many also have a potential fishery importance. The Atlantic bumper, Chloroscombrus chrysurus, in the southern Gulf of Mexico is one such example where it is one of the most abundant, unexploited species (Yañez- Arancibia and Sánchez-Gil, 1986; Flores-Coto and Sánchez-Ramírez, 1989; Tapia-García, 1991). Atlantic bumper spawns in Campeche Bay year round but mainly in spring and summer, in areas less than 40 m deep; the highest larval abundance has been recorded off Términos Lagoon (Flores-Coto and Sánchez-Ramírez, 1989). Except for descriptions of larval abundance and distribution in the southern Gulf of Mexico, there is no information on the early life history of this species. Therefore a better knowledge of Atlantic bumper is a priority and thus the goals of this study are to estimate ages of its larvae and to determine larval growth and mortality rates by season. MATERIALS AND METHODS LA RVA L COLLECTION.—The study area is located in the southern Gulf of Mexico between 18°06'– 21°00'N and 90°26'–97°20'W. It comprises the continental shelf and the adjacent oceanic zone of the States of Veracruz, Tabasco and Campeche (Fig. 1). The zooplankton samples were collected aboard the oceanographic vessel JUSTO SIERRA, during 13 cruises between 1984 and 1993 (Table 1). Zooplankton sampling consisted of a double oblique 295 296 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 Figure 1. Study area plankton tow following a circular course using a bongo net with 333- and 505-µm mesh nets, except for the cruise Mopeed V where several depths in the water column were sampled using an opening- closing net of 75 cm diameter and 505-µm mesh. The filtered water volume was calculated using flowmeters placed in each net. Larvae were sorted out of 505-µm mesh and preserved in 70% ethanol. Larval Atlantic bumper were measured as standard length, or notochord length before notochord flexion, with 0.1 mm precision. AGE AND GROWTH.—For growth estimation, sagittal otoliths were obtained from larvae collected in the summer 1987 and 1988, winter 1992 and 1993, and spring 1992 cruises. Because of the poor condition of the otoliths in the fall 1987 cruise and the scarcity of larvae on the fall 1992 cruise, no larvae were aged from the fall samples. SÁNCHEZ-RAMÍREZ AND FLORES-COTO: GROWTH AND MORTALITY OF ATLANTIC BUMPER 297 .ocixeMfofluGnrehtuosehtotsesiurchcraeserfoyrammuS.1elbaT Ceesiur DnatSseasoNumberofstation I4MECO1r5–25February198w9inte2 P4ROGMEXII2g5April–4May198s9prin3 P4ROGMEXIII7r–17August198s5umme5 O7GMEXI2r5February–9March198w1inte5 O7GMEXII2r7July–5August198s8umme6 O7GMEXIII2l8November–5December198f4al4 O8GMEXV1r–9August198s6umme7 O9GMEXVII1r2–16February198w2inte4 M2OPEEDI1r3–16February199w3inte2 M2OPEEDII2g0–24June199s1prin2 M2OPEEDIII1r0–22September199s7umme1 M2OPEEDIV7l–17November199f2al2 M3OPEEDV1r2–18February199w0inte2 Age of larvae was estimated by counting the number of growth increments on sagittal otoliths and adding a value representing the number of days from hatching to formation of the first growth mark. This aging technique for larval Atlantic bumper has been validated by Leffler and Shaw (1992) who consider that each observed growth increment in the sagittal otolith represents 1 d in the larvae of this species. Some otoliths, particularly the largest ones, could not be read. To solve this problem, the long and short radius, and the diameter of all otoliths were measured, thus estab- lishing relationships with the number of increments of those that were readable. For growth models of each season (spring, summer and winter) a linear model was fitted to the age/length data since it had the highest determined coefficient (r2): SL = b(t) + a Eq. 1 where: SL = Standard Length or notochord length (mm); a = y-axis intercept; hatching size, mm (size at 0 age); b = constant, growth coefficient; and t = age of the larvae, expressed as the number of daily growth marks plus number of elapsed days, from hatching to the formation of the first growth mark. MORTALITY.—To build the mortality model it was necessary to estimate larval abundance. The abundance of larvae at each sampling station was standardized as number of larvae m−2, as pro- posed by Houde (1977), in this case for each 0.5 mm size class, using the model: Nij = cijdi/vi Eq. 2 −2 where: Nij = number of larvae m of marine surface at station i, of size class j; cij = caught larvae at 3 station i, of the size class j; di = depth of tow (m) at station i; and vi = filtered water volume (m ) at station i. Average larval abundance for each size class (0.5 mm SL) for each season was estimated for the different cruises: two for spring, four for summer and five for winter: n A = ∑ N /N Eq. 3 j i=1 ij ps 298 BULLETIN OF MARINE SCIENCE, VOL. 63, NO. 2, 1998 Figure 2. Relationship daily increments-radius otholits of larval Chloroscombrus chrysurus. (A) spring; (B) summer; (C) winter. Southern Gulf of Mexico. where: Aj = Larval average abundance of size class j; Nij = defined above; and Nps = number of positive stations by season s, corresponding to the stations with Atlantic bumper larvae. Mean age of each size class was estimated through the growth models for each season. The instantaneous mortality rate (IMR) was estimated from an exponential model that described the decrease in the average abundance of each size class: −zt Aj = Ajoe Eq. 4 −2 where: Aj = Larval average abundance m at age t; Ajo = constant. This intercept is an estimation of larval abundance at age “0”; z = instantaneous mortality rate (d−1); and t = age (in days). Mortality (A) and survival percentage (S) were obtained from the models: A = 100(1−e−z) S = 100e−z S = 100−A, 0 ≥ S, A ≤ 100 Eq. 5 For the mortality analysis, we had to first establish the size at which larvae were captured by the sampling gear to eliminate from the model small larvae that could pass through the mesh, and consequently not be well represented in the catch. We measured 109 larvae to establish the standard length and body depth relationship. Considering that the maximum opening area in the 505-µm mesh is 0.714 mm, larvae ≥2.3 mm SL (0.8 mm body depth), were captured by the sampling gear. Because average larval abundance in the spring showed a lower value in the 2.3 mm size class, than in 2.8 mm; the Robson and Chapman method (1961) was used to determine if inclusion of the 2.3-mm size class was valid in the mortality model. The largest sizes were probably not well sampled because they evaded the net. Therefore, starting with the first size class, where no larvae were captured, they were eliminated. SÁNCHEZ-RAMÍREZ AND FLORES-COTO: GROWTH AND MORTALITY OF ATLANTIC BUMPER 299 Figure 3. Growth of larval Chloroscombrus chrysurus. (A) spring; (B) summer; (C) winter. Southern Gulf of Mexico. RESULTS AND DISCUSSION GROWTH.—To estimate the number of growth marks of otoliths in which it was not read, and to include these larvae in the growth models, the analysis consisted of the relationship among the number of daily increments and shorter, longest radius and diameter of the otoliths. The highest coefficient of determination in the linear model was found with the longest radius. Larvae of size 1.6–6.6 mm SL from spring, 1.4–6.0 mm SL from summer and 1.9–7.2 mm SL from winter were used in these relationships (Fig. 2). For the growth models by season, the size ranges used were: 1.6–6.6 mm SL (57 larvae) in spring, 1.4–11.7 mm SL (69 larvae) in summer, and 1.9–7.9 mm SL (54 larvae) in winter. As mentioned above, the linear model better fitted the observed data. Estimated growth rates were higher in spring and summer (0.17 mm d−1) than in winter (0.12 mm d−1) (Fig.

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