Diet of Agonostomus monticola (Pisces: MugHidae) in the Río Ayuquila, Sierra de Manantlán Biosphel'c Reserve, México

Abstrae!: We quantified the diet of Agonostomus monticola during the wet and dry seasons o f 1990 in a 15 km reach o f the Río Ayuquila, a Pacific tributary o f west-central México. The species consumed 32 animal and 9 plant families, with aquatic insects (mainly Diptera, Ephemeroptera, and Trichoptera) and algae (mainly Chlorophyta) as primary prey. Animal material dominated the diet in the wet season but was only slightly more important than plant material in the dry season. Animal material increased in importance with fish size for specimens 151-300 mm total length. Río Ayuquila A. monticola ate fewer shrimp, Odonate insects, fish, and froit than other studied populations. The broad diet of A. monticola suggests that it is an opportunistic feeder

Agonostomus monticola (Mugilidae) is an important food fish in the rivers of the Sierra de Manantlán Biosphere Reserve, west-central México, as well as in many other parts of tropical America (Lyons andNavarro-Pérez 1990, Santana-C. et al. 1990).However, A. monticola populations in the Manantlán reserve and elsewhere are declining, and additional inforrnation on the species life history and ecology is needed to aid in conservation.In tbis paper we characterize the diet of A. monticola, known locally as the "trucha de tierra caliente", in the Río Ayuquila, the largest river in the Manantlán reserve.A companion study (Navarro-Pérez 1992) examined other aspects of the biology of A. monticúla in the Río Ayuquila.
Sorne data on the feeding of A. monticola are available from Costa Rica (Winemill er 1983, Bussing 1987), Honduras (Cruz 1987), Trinidad (Phillip 1993), and the United States (Loftus et al. 1984).Agonostomus monticola has been characterized as either an insectivore (Winemiller 1983, Cruz 1987) or an omnivore, feeding on both algae and aquatic invertebrates (Loftus et al. 1984, Bussing 1987, Phillip 1993).However, sample sizes were smaU « 5 stomachs) for all but the Cruz (1987) and Phillip (1993) studies.Moreover, published data may not be completely applicable to Mexican populations, as what is currently referred to as A. monticola could represent a complex of at least two species (Miller 1995).Populations in the Manantlán reserve may be both taxonomically and ecologically different from previously studied populations, and specific data from reserve populations are needed.Field sampling: We sampled for nine months in 1990, four in the dry season (November, February, March, and April) and five in the wet season (May, July, August, September, and October).Each month, digestive tracts of freshly captured A. monticola were obtained from local fishermen.They fished between 06:00 and 22:00 hours and captured fish by cast net in the dry season and hooks with worms in the wet season.Digestive tracts were removed from captured fish and immediately preserved in 5% formal in for later examination in the laboratory (Korschgen 1987).Total length and weight were recorded from captured fish.
Laboratory analyses: All food organisms were identified (Alvarez del Villar 1970, Barnes 1977, Wiggins 1977, Merrit and Curnmins 1978, Hurlbert et al. 1981, Ortega 1984y Burch 1987) to family and counted and weighed, and then their volume was estimated by water displacement.For each food item up to five diet indices were calculated (Hyslop 1980): percentage number (N; number of individuals of food item divided by total individuals of all food items), percentage dry weight (W), percentage volume (V ), frequency of occurrence (F; number of stomachs with item divided by total number of stomachs examined), and index of relative importance Index values were compared between length groups and season with analysis of variance and Duncan Multiple Range te s ts (P> 0.05).
A total of 140 specimens of A. monticola were examined during the study.These fish ranged from 92-428 mm total length, with a mean total length of 211 mm and weight of 128 g.Digestive tracts were obtained from 135 specimens.The digestive tract consisted of mouth, esophagus, stomach, two pyloric cacae, intestine, and anus.The inside wall of the stomach typically had thick, parallel folds, although in a few cases these folds were irregular or absent.One oí the pyloric cecae was always longer than the other, and both were usualIy lined with thin irregular folds.
The diet of A. monticola differed significantly between seasons.In the wet season (43 stomachs), animal material was significantly greater than plant material in N (73%), V (55%), W (61 %), and IRI (78%).In the dry season (92 stomachs), animal material was equal to plant material in N (50%), V (48%), and W (49%), but greater in IRI (61 %).Insects were the most frequently encountered food items in both seasons, with F of 52% in the dry and 42% in the wet season (Fig. 1).The m ost common families were individuals eating relatively more animal material.We established eight 50 mm total length classes between 51 and 450 mm.However, sample sizes were sufficient for analysis only in the three size classes from 151 to 300 mm.For the 151-200 mm group (46 stomachs), plant material was greater by N (51%), V (80%), and W (78%).In the 201-250 mm group (63 stomachs), animal material was greater by N (55%) but plant material was greater byV (56%) and W (58%).In the 251-300 mm group (14 stomachs), animal material was greater by N (77%), V (58%), and W (62%).In the Río Ayuquíla, A. montícola feeded somewhat more on animal than plant material.This findíng was consistent with results from previous studies (Loftus et al. 1984, Bussing 1987, Phillip 1993).In the Río Ayuquila, the species consumed a wide mnge of organisms, with aquatíc insects and algae dominating.The importance of algae in the diet was lowest in the wet season, when scouring flows and high turbidity after rainstorms likely reduce the amount of algae available for con��mptíon.Phillip (1993) also found algae to be less common in the diet in the wet than the dry season in Trinidad.
The diet of Río Ayuquila A. monticola was different in several ways fram the diets of the two other populations that had been well studied.Cruz (1987) characterized A. monticola fram the Río Plátano on the Atlantic slope of Honduras as primarily insectivoraus.He examined many more small « 100 mm) specimens than we did, but for his largest size category (110-215 mm standard length), he found plant material to have a V of only 9% compared to 56-80% for this size category in the Río Ayuquila.His A. monticola also consumed substantial amounts of shrimp (Crustacea; V = 20%), fish (Characidae; 18%), and Odonate insects (17%).These taxa occurred in the Río Ayuquila (Navarro-Pérez 1987, Lyons and Navarr o-Pérez 1990) but were absent or ínfrequent in the diet of A. monticola there (F < 2%).In the Shark River, Trinidad, Phillip (1993) found the most frequently encountered diet items in the wet season lo be insects (F = 52%), shrimp (35%), fruit (32%), and algae (15%).In the dry season the diet shifted to algae (66%), insects (24%), fruits (18%), and shrimp (13%).We found fruits to be ínfrequent in the diet of A. monticola in both the wet (3%) and dry season (l %).
The food habits of A. monticola in the Río Ayuquila allows exploitation of a wide range of food resources and should minimize the effects of shortages of a preferred item or competitíon with other species (Phillip 1993).A. monticola appears to be an opportunistic feeder, and this should suggests the lack of strong habitat specificity, which has been confirmed by Navarro-Pérez (1992), who found the mullet in a wide range of habitats in the Río Ayuquila.The ability to use a wide range of food resources and habitats should make A. monticola relatively tolerant of environmental variation.Food items found in stomach of A. monticola during diff erent sampling monfhs Others plants: Acnatic fanerogama Podostemaceae X X

Superior plant
Leguminosae X X Study area: AH data were collected from the Río Ayuquila, which forrns the northeastern boundary of the Sierra de Manantlán Biosphere Reserve (RBSM) in the state of Jalisco.The Río Ayuquila originates about 100 km north of the reserve and flows along the reserve for about 40 km before joining with the Río Tuxcacuesco to form the Río Armería, which flows approximately another 150 km before emptying into the Pacific Ocean.The river in the reserve experiences a tropical climate, with a mean annual air temperature of 28°C and a water temperature of 26°C (Torres-Navarr o 1994).There is a wet season from May through October, and a dry season from November through April, with almost no rain from February through April.The study area encompasses 15 km of the river between the villages of Guamuchil (104° 01 ' W 19° 35' N) and Ve ntanas (104° 06' W 19° 41' N).The river here has a mean width of 31 m and a mean thalweg depth of 1.1 m, with numerous pools and rapids.Substrate is mainly boulder and rubble, with sorne sand and grave!.
Fig. 1: Frequency of occurrence of different food iteros of A. montico la.