Life history strategies, population regulation, and their implications for fisheries management

Kirk O. Winemiller

Life history theories seek to explain the evolution of organism traits as adaptive responses to environmental variation.  Life history theories also make qualitative predictions about population responses to natural and human-induced disturbances at different spatial and temporal scales.  A model involving three primary life history strategies, or syndromes, predicts population responses to patterns of natural and anthropogenic disturbance at variable scales of time and space.  The model provides a general means to predict a priori the types of populations that should have high or low demographic resilience, production potential, and conformity to density-dependent regulation.  For certain species and habitats, the model challenges the notion that population dynamics can be predicted based on density-dependent responses to environmental carrying capacity.  Periodic (long-lived species with high fecundity) and opportunistic (small, short-lived species with high reproductive effort) strategists should conform poorly to projections from models that assume significant density dependence over long time scales.  The model's implications for fisheries management are explored relative to sustainable harvest, endangered species conservation, exotic species, habitat suitability indices, supplemental stocking of marine fishes, artificial reefs, and marine reserves.  The degree of compensatory reserve, population resilience, and conformation to equilibrium assumptions of population dynamics should vary predictably based on life history attributes, which provides guidance for management, especially when detailed information is lacking. 

Resource Use Among Sympatric Lepisosteidae Species in the Middle Brazos River and Associated Oxbows

Clint R. Robertson, Steve C. Zeug and Kirk O. Winemiller

This project examines habitat and diet partitioning among three sympatric gar species (Lepisosteus osseus, L. oculatus and Atractosteus spatula)  in the middle Brazos River and associated oxbow lakes.  We are comparing species relative abundances among the river channel (lotic habitat) and two oxbows that form connections with the river channel at different water levels (lentic habitats).  Fishes are collected using experimental gill nets and standardized beach seine hauls.  Preliminary results indicate that L. osseus is more abundant in channel habitats, with few collected in oxbow habitats.  L. oculatus is more abundant in oxbow habitats, with few collected in channel habitats.  A. spatula was far less frequently collected than the other two species, with one individual caught in the channel and another in an oxbow.  Preliminary diet analysis (volumetric stomach contents analysis) indicates high variability among channel and oxbow habitats, with the diet of L. osseus in the channel dominated by fishes of the families Ictaluridae and Cyprinidae, while the diet of L. oculatus in oxbows is dominated by fishes of the families Centrarchidae, Cyprinidae and Clupeidae.

Ontogenetic, seasonal, and spatial variation in the diet of Heterotis niloticus (Osteoglossiformes; Osteoglossidae) in the Sô River and Lake Hlan, Benin, West Africa

Alphonse Adite (Univ. Abomey-Calavi, Benin), Kirk O. Winemiller and Emile D. Fiogbe (Univ. Abomey-Calavi, Benin)

The African bonytongue, Heterotis niloticus, is an important species in fisheries and aquaculture in West Africa.  This osteoglossid has frequently been characterized as a detritivore, in part because of its benthic feeding habitats and possession of a gizzard (thick-walled pyloric stomach).  Diets of two populations in the Sô River in Southern Benin were examined over 18 months (1,461 specimens).  A population from the river channel and seasonally flooded marginal plains was dominated by juvenile and subadult size classes.  Adults, including reproductively active individuals, were common in second population from Lake Hlan, a natural lake in the river floodplain located 35 km upstream from the channel study region.  Heterotis of all sizes consumed a variety of food resources, ranging from aquatic invertebrates to small seeds.  Aquatic invertebrates composed a large proportion of the diets of juveniles, and adults consumed a mixture of aquatic invertebrates, seeds, and detritus.  Seasonal dietary variation was observed in both populations, and diet breadth was not significantly different between populations.  Individuals <100 mm SL fed primarily on aquatic invertebrates. Aquatic invertebrates remained significant in the diet of larger size classes.  Diets of larger fish began to include seeds and detritus, with a marked increase in the volumetric proportion of detritus in the diet between 300 to 400 mm in Lake Hlan and between 500 and 600 mm in the river.  Relative gut length was inversely related to body size, which supports the notion that Heterotis is an omnivore and not a specialized detritivore.  The thick-walled gizzard of Heterotis, which generally contained sand, probably aids digestion of seed coats.

Population structure and reproduction of Heterotis niloticus (Osteoglossiformes: Osteoglossidae) in the Sô River-floodplain system (Benin, West Africa)

Alphonse Adite (Univ. Abomey-Calavi, Benin), Kirk O. Winemiller, and Emile D. Fiogbe (Univ. Abomey-Calavi, Benin)

Reproductive ecology of the African bonytongue, Heterotis niloticus, was examined in the lower Sô River, its flooded plains, and Lake Hlan, a flooplain lake located approximately 60 km upstream from the river study sites in southern Benin.  Both locations support important fisheries in which Heterotis is the principal target species during the flood period.  Ripe and nesting adults were common in Lake Hlan, whereas only 3.5% of individuals captured from river sites were adults.  Monthly averages for the gonadosomatic index and the percentage of individuals with mature gonads peaked as water levels increased during the wet season (May-August) then gradually declined during the flood period (September-November).  Frequency distributions of oocyte size classes within ripe ovaries indicated production of multiple cohorts per spawning season.  Median maturation size was estimated at 675 mm TL for both genders.  Fecundity (number of mature oocytes in the ovary) increased with body size.  Lowest and highest fecundities recorded for individual fish were 2,697 (500 mm SL) and 27,508 oocytes (735 mm SL), respectively.  Averages ranging from 37-51active nests per hectare per month were observed in Lake Hlan during the peak spawning period (May-August).  The number of larvae per nest ranged from 3,953 to 6,125.  At 5-7 days posthatch, larvae averaged 13.10 ± 0.51 mm SL and 15.79 ± 1.09 mg.  Yolk sacs were resorbed at 6-7 days posthatch.  At 5-6 days posthatch, larvae formed swarming schools during daytime, and at 25-30 days posthatch fry foraged independently.  Lake Hlan bonytongues constitute an important source subpopulation that exports individuals to river/floodplain areas downstream where intense fisheries harvest almost entirely juveniles and subadults.

Effects of seasonality and migratory prey on body condition of Cichla species in a tropical floodplain river

David J. Hoeinghaus, Kirk O. Winemiller, Craig A. Layman, D. Albrey Arrington and David B. Jepsen (Oregon State Univ.)

The Cinaruco River of southwestern Venezuela is a temporally dynamic floodplain system with uncommonly high densities of large piscivores of the genus Cichla.  Three Cichla species naturally occur in this system, and are known to exhibit significant spatial and temporal resource partitioning.  Resource availability in the Cinaruco varies in response to seasonal changes in water level.  The prochilodontid Semaprochilodus kneri migrates en mass from the Orinoco floodplain into the Cinaruco system during the falling-water period, and is potentially an important prey of larger-sized piscivores.  In the present study we investigate patterns of variation in body condition of Cichla among species, seasons, and two size-classes.  Morphological characteristics were used to determine the size class division a priori, representing the threshold after which Cichla can consume Semaprochilodus based on gape-limitation.  Significant seasonal differences in the intercepts of the log-transformed length-weight relationships were observed.  Lowest body condition was observed during the rising-water period for all three species, whereas only C. temensis had significantly higher body condition during the falling-water period than low-water period.  Comparison body condition among size classes within species resulted in a single significant difference: large C. temensis had significantly greater condition during the falling-water period compared to smaller individuals.  Our results correspond with previous studies on Cichla ecology in the Cinaruco River, which describe the importance of S. kneri in the diet of large C. temensis only (comprising more than 50% of ingested prey), and numerically characterize the increase in body condition experienced by large C. temensis as a result of this subsidy.

Updated Monday February 27, 2006