Lake and Stream Fishes: Ecology, Adaptation, Diets and Resource Use
Keast, James Allen
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This text has as its objective a synthesis of the ecology and biogeography of small lake fish systems, as illustrated by the North American fauna and, specifically, the cool temperate one of the northeast of the continent. Freshwater systems are ancient. They go beyond the Paleozoic. North America is a water rich and diversified continent, with latitudinal range from the Arctic to near the Tropics. A fossil record of fishes going back to the Ordovician testifies for a long period of fish evolution with an increasing diversity of body forms for freshwater fishes over time. Water has distinctive attributes. With high viscosity and density, it provides physical bodily support for its inhabitants and buoyancy against gravitational pull. Aquatic organisms live suspended in their environment. Light penetrates water poorly, meaning that most productivity is near the surface. Oxygen is optimal near the surface. The best habitat is along the shoreline and near the surface. Cold temperature is a great limitation to fish occurrence and activity. In the north, fish are relatively inactive in winter. Patchiness is a big factor in the distribution of aquatic organisms, as on land. Benthic invertebrates, all important as fish food, relate to cover, substrate (mud, sand, rock), particle size and diversity, grain spacing, organic content, and vegetation height and type. Fish species may be habitat specialists (for example, confined to cover), or generalists utilizing multiple habitats. Age classes within species often use different habitats. This is a very important factor of habitat use by fishes. The characteristic body shape of fishes, fusiform, is demonstrably the most efficient shape for passing through the aquatic medium. Absence of legs limits overland movement between water-bodies. Body morphology and all other systems, physiologies, annual cycles, reproductive methods, and ecologies, are specially adapted to the aquatic environment. Different body shapes, fin positions, mouth sizes, angles of mouth opening shapes and positions on the head, marginal and pharyngeal dentitions, gill raker numbers, alimentary structures, vision, auditory, and lateral line variations combine to form adaptive suites. Some of the features have a physiological, in contrast to anatomical, role. Adaptive features in North Temperate freshwater fishes do not include the more extreme morphologies found in the sea, in coral reefs, and in African Rift lake fishes. North Temperate fish differ from homeothermic terrestrial ones in that there is limited annual activity to perhaps six months, and species are represented by multiple size (age) and ecomorphological types. Growth being limited to a few months a year means that fish species are represented by series of distinct age (size) cohorts, which differ somewhat in ecology. Results of diet studies involving the prey resource base indicate (1) Diets are chosen from widespread and predictable prey types, the range is limited. (2) Each species has an individual diet. (3) Diets tend to be framed around a small number of basic prey types: these characteristically make up a significant proportion of the diet. (4) Body morphological differences tend to favor this. (5) Opportunistic feeders seasonally shift diet to more (abundant) prey types. (6) In generalist feeders, some prey items are represented in diets as small numbers. These types are presumably taken as encountered when foraging. (7) "Generalist" and "specialist" feeders (the latter defined by using only a few types) co-occur. The classification is artificial: a species may be a "generalist" in taking many taxonomic types but a "specialist" in body size. Simultaneous sampling of the zoobenthos and zooplankton shows that most seasonal diet shifts in fish species is directly linked to prey abundance. The peaking abundance of a prey type will persuade many fish species to shift to it. (8) Seasonal diet shifts are linked to availability of the different prey types. (9) Absence of a "preferred" prey type may lead to inferior size cohorts. (10) Herbivory is uncommon in North Temperate fishes, in contrast to the Tropics (Chapter 11). Sometimes the herbivores ate animal matter at the beginning of spring following the long winter period of food deprivation. (11) There is a good general link between mouth and prey size. (12) Ontogenetic diet changes occur in all fish. They are marked in the larger-growing and longer-living species. (13) Age classes of species commonly differ in proportions of a few prey types eaten, with the selection of larger-bodied individuals as fish grow. (14) Differences in diet (as in rock bass in Opinicon - see Chapter 6) may be as marked as those otherwise separating different species. (15) Diet overlaps, calculated on proportions of different prey types eaten, are usually high. In almost all cases, increased overlaps occurred when a prey type became very abundant. The fish species were attracted to an item rather than being forced to utilize it by shortage. (16) Abundances of species in systems vary greatly. In Opinicon, bluegill made up 70% of total abundance and biomass of species, while some other species made up less than 1%. The different fish species thus varied greatly on impact on the potential prey resource base. (17) This largely undermined any argument that species diets were molded by interspecific competition. There was not any "tight" resource division in this "open" system. (18) No evidence was found of individual fish within a community or groups showing preference for particular types. Characteristically, all individuals of a group took whatever was abundant. (19) Fish species taking large prey ate much fewer items than small prey eaters, cropping patterns were necessarily different, the former using their energy to find fewer, rarer prey. These generalizations were confirmed from studies of a wider range of lakes and streams (Chapters 6, 7, 8). They presumably represent the optimal use and division given the ecosystem features. Fish larval complexes are made up of large numbers of minute individuals that appear over a relatively brief period annually at the same time year to year. Their impact on the ecosystem is temporally potentially very high but abundances fall rapidly with mortality, dispersal, and growth. There is a sequencing in time of spawning of different species in most systems. This, and the separation of many species into separate spawning bouts, spreads impact on the prey resource base and limits intraspecific and interspecific competition. Major features of larval ecology in freshwater lakes are (1) Larvae are released over a period of several weeks in spring when abundance of the prey types is high. (2) Feeding may begin immediately or, in larger eggs, be delayed for a few days. (3) Within days, growing from body lengths from 4-5 mm to 8-12 mm, increased mouth size initiates a shift to prey of larger size (for example, larger bodied copepods) and the cropping of a more diversified range of prey. (4) This change (and early dispersal) now reduces diet overlap in the hatchings of previous ovulations (in bluegill, for example, spawnings may be 7-10 days apart). (5) A great number of predator types attack larvae. With growth, predation falls. Dispersal is rapid. Interspecific competition as a mortality factor remains inadequately quantified. (6) In northern lake systems, the typical pattern is for larvae to hatch in the inshore open water littoral zone, and as young juveniles, they move into the macrophyte beds. In a few species, hatching occurs in macrophyte beds. (7) In the first weeks of life, the larvae and early juvenile of the species go through a sequence of equivalent diets: the large-mouth bass, that hatches with a large mouth, however, misses the copepod feeding phase and starts with small insects. (8) Sequential spawning of species expresses phylogeny and temperature physiologies. (9) Capacity to shift diet between alternative diets increases as the young juveniles start to grow. Studies of yellow perch in Oneida Lake, New York, confirm much plasticity in diet after the first weeks. (10) "Adult type" diets are mostly achieved in the first summer of life, but pumpkinseeds, dependent on development of molluskivore pharyngeal dentition, may not achieve this until the second summer. (11) Importance of achieving "maximal" size during the first summer of life, and ahead of the winter, has been recorded by many workers. (12) The larvae of different species vary in different ways (size of eggs, size and time of hatching, body shape). (13) Amount of growth is limited by seasonal water temperature in the North Temperate zone. (14) Within a hatch, young fish (for example, young largemouth bass) exhibit depensation and compensation relative to diet factors. (15) Larval-early juvenile success has a major factor in determining age classes of fish. Ontogenetic diet shifts to energetically more beneficial prey types are demonstrably critical to good growth. The availability of a sequence of prey types of increasing body size and energy content is required to support this, or growth may be limited. Prey categories that are abundant and diverse (for example, zooplankton, fish, detritus) are cropped by many fish species. By contrast, seasonal availability of other foods like odonates and ephemeropterans, chironomids and Trichoptera larvae is too limited or numbers are too small to be widely used. The guild concept in fishes is of limited significance in fish ecology. To explore dietary consistency, a series of contrasting 12 lakes fish types was selected and the diets of their component fish species analyzed. It was found that (1) For generally "similar" lakes (in this case, mid-sized, diversified systems) the wider-spread species analyzed had comparable diets. They differed mainly in proportions of similar prey types eaten. (2) For contrasting lakes with different fish species compositions, diets and resource division patterns differed markedly from each other. (3) Between lakes, diet differences were not random. Commonly the shift was to a dominant resource that, in another study lake, was the second or third most important diet item. The shift was sometimes associated with the absence of a usual dominant fish predator of that type. Thus, pumpkinseed and banded killifish became the main chironomid feeder in the absence of the bluegill. The yellow perch that commonly has a characteristically ontogenetic diet shift of small invertebrates to large invertebrates to fish became a zooplanktivore in Sunfish Lake where there was a high population of the large-bodied zooplanktor Daphnia. Perch became precocious piscivore in Atkins Lake where small-bodied fish prey were abundant and other piscivores uncommon. The changed diets were mainly related to different resource bases. The diet changes were facilitated in three ways by (i) an item that was an incident diet type in one system becoming the dominant form in another; (ii) an ontogenetic stage in one area becoming the dominant; and (iii) different resource bases inviting the change. There was a limit in how severe a diet change could be. Prey size, status in the particular system, and body form were also factors. Lakes and streams are distinct as life zones. Both provide great variability. Expanse, degree of lineation, morphology, total depth, depth diversity, thermal regimes, habitats available, and quantity of fish faunas vary. Major features of feeding and diets in stream fishes are as follows (1) Major prey types in streams are chironomids, ephemeropterans, Trichoptera larvae, drift organisms, and detrital material. (2) Swift-running small streams with limited bottom cover and backwaters provide only limited resource bases. Sluggish (often secondarily degraded), macrophyte-dominated streams have a wider diversity of potential prey types. In the former, fish diets are relatively limited. In the latter, they are diversified. (3) In highland generated Medway Creek, Ontario, the entrant drainage area is populated by small-sized species and age classes, with fish species grouped into chironomid specialists, detritivores, and one generalist insect-eater. This resource use and division pattern also characterized downstream sites but with herbivores becoming less important. Generalized insect-eaters and Chironomid fish specialists, which included most darters, were major faunal components in downstream areas. Diet overlaps were higher than in most of the lakes studied. Ontogenetic diet changes were minor except in the larger species. (4) Sluggish, lowland, macrophyte-dominated Jones Creek had diversified invertebrate faunas and fish diet diversities. (5) As an example of an impoverished stream diet, Shelter Valley Creek fish ate chironomid larvae, Trichoptera larvae, and Ephemeroptera nymphs. Ontogenetic diet shfits were minimal. Zooplanktivores, and detritivores and, as in the Medway, piscivores, were absent. (6) In Lower Poole, a still-water, a beaver pond with mud stratum, chironomids formed the major food. Major distinctions between tropics and North Temperate regions (Chapter 11) include (1) High number of species, commonly double of that in equivalent temperate areas. (2) A great diversity of adaptive morphological types. (3) Additional feeding "guilds". Besides zooplanktivores, benthic insectivores, molluskivores, piscivores and herbivores (the latter two categories highly diversified), there are fruit eaters, leaf choppers, and scale, fin and eyebiters (Fryer and Iles). (4) Habitats (for example, rock substratum) support whole faunas, not just a few species. (5) Distinctive morphological types and space divisions are major mechanisms permitting multiple species occurrence. In big tropical rivers, additional habitat types (for example, floating vegetation) support distinctive fish assemblages. There are very large numbers of piscivore species in the African Rift lakes. The great tropical rivers are characterized by major seasonal habitat change associated with annual flooding. There may be huge fish dispersals associated with this. A protracted growth season means that there is not the great diversity of ontogenetic forms found in the north. Evolutionary rates may be very rapid in the tropics. The basis of this is still not adequately understood (Chapter 11). Sexual selection is seen to be involved. Species swarms involving single lineages are found in Malawi, Tana, and in the Cameroun volcanic lakes. Ecological systems in their nature and orderliness suggest structuring, organization, repetitiveness, and predictability. No subject of ecology, however, is so elusive as determining whether measurable structure occurs to explain biomass, seasonal variation, species richness, ecological roles, and resource division patterns. Three potential structural systems may apply: competitive exclusion (space use and diet), energy flow, and food webs. They emphasize different components. Competitive exclusion has been area of study since the 1950's. The question of whether assemblies are controlled by species interactions or the physical environment has long been joined. As a contrast, a hierarchical concept of energy flow has been advanced as the "currency" of ecosystems. All three concepts suffer from sloppiness and inadequacy. If there is no simple structuring base to be found, why are systems not just random? The answer is, obviously, that the ecosystem does dictate certain repetitive viable patterns. The rest of the answer possibly lies in: (i) complexity; and (ii) adaptability and plasticity. Natural systems have many in-built alternative adaptive responses to mitigate environmental uncertainty. Therefore, space use and trophic systems function in a framework with guidance and general rules, not rigid and tight ones.