CHAPTER 1
Introduction to Aquatic Ecosystems

The organisms that scientists consider fishes represent the most diverse groups of all vertebrates as there are more species of fishes than of all other vertebrates combined. Fishes occupy diverse habitats such as normal surface waters, great ocean depths exceeding 8,000 m, heated desert pools and caverns that may exceed 40 °C, caves deep in the Earth, under the ice in the Arctic and Antarctic seas, and a variety of other extreme habitats. Adaptation to these habitats has resulted in extreme diversity in fish physiology and anatomy, coupled with differences in foraging patterns and other behaviors. This variability makes it hard to generalize the responses of fishes to local conditions but makes their adaptation to environments very interesting.

There has been much recent debate about what exactly is a fish. Historically, fishes have been considered to include five classes of vertebrates, although the taxonomic status of fossil groups is unsure. Currently existing fishes represent three classes: Agnatha - hagfish and lamprey; Chondrichthyes - cartilaginous fishes including sharks and rays; and Osteichthyes - bony fishes including the most current species. A recent novel by Miller (2021) has examined not only the history of fish biology and systematics but also the evidence that fishes do not represent a single evolutionary line, which means they have not evolved from a single common ancestor. Genetic evidence has recently shown closer relationships between some of the groups of fishes and reptiles, amphibians, and mammals, questioning whether what we call fishes actually are an evolved group or are just an animal life form. Ecologists have included all aquatic living vertebrates with gills, scales, and fins as fishes (although some species have secondarily lost some of these traits), and we will follow that pattern in this book. Certainly cladists (evolutionary biologists that study species relationships) can better explain the evolution of different classes of vertebrates, which may change this higher level of taxonomy. For the purpose of this book, we will continue to consider the three classes of vertebrates listed above as fishes as they share similarities in their life form.

Freshwater organisms are among the most endangered of all species in the world. Analysis by The Nature Conservancy shows that 70% of all freshwater mussels, 50% of crayfishes, and 40% of freshwater fishes are at risk of extinction in the United States (Master et al. 1998). In comparison, approximately 18% of reptiles, 15% of mammals, and 14% of birds are in a similar status. This is particularly daunting when we realize that the highest diversity of vertebrates is found in the classes known as fishes, where there exist at least 32,000 species. This is more than all the species of birds, mammals, reptiles, and amphibians, combined. Such a high fraction of the fauna being endangered among freshwater fishes is due to the various challenges we place on freshwater ecosystems, including the use of water for irrigation, industry, and human consumption, as well as the discharge of chemicals into water for disposal. In addition to direct use of water, humans alter habitat by building dams, channelizing streams for ship passage, and building canals. All of these changes in aquatic ecosystems have resulted in major reductions in the fish fauna. There have been a number of evaluations of factors causing animal extinction in various ecosystems. All of these divide the causes of extinction into three main groups of approximately the same magnitude: introduction of exotic species, overexploitation, and habitat disruption. Since fishes are the only major group of organisms remaining that are hunted as food on a global scale, much damage is due to overexploitation, as well as habitat disruption and exotic species. It is no wonder why freshwater organisms, in particular freshwater fishes, are under such threat.

Ecology has a variety of popular, or lay, definitions, but the science of ecology has been well defined and accepted by most scientists. The definition has evolved over time, depending largely on the level of our understanding of ecological interactions. Krebs (2009) provided the best definition: ecology is the study of the interactions that determine the distribution and abundance of organisms. These can be categorized as interactions with physical, chemical, or biological factors in the environment. The purpose of this textbook is to overview the means by which fish distributions and abundances are influenced by physical, chemical, and biological factors.

This book is divided into six main topics that focus on the three major disciplines of ecology: physiological, behavioral, and community ecology. These three disciplinary areas of ecology have boundaries that are intentionally unclear, so some concepts will be presented several times throughout the book.

To appreciate the ecology of fishes, it is important to first understand the habitat in which fish exist - the aquatic system. This chapter reviews living in the water, the characteristics of fish, and aquatic ecosystems, emphasizing several systems that are more familiar. A key theme throughout this book (highlighted in this chapter) is that the environments in which fish exist differ in two important dimensions: the distribution of temperature in time and space, and the distribution of food in time and space.

Properties of Water

Water has a number of physical properties that are challenging to organisms living in the water and yet promote life within the water because of the long-term stability of water conditions. Water is one of the few compounds that is liquid at ambient temperatures and has high viscosity and surface tension. This means that movement through the water is difficult, and diffusion across the water surface level is limiting. Animals moving within the water must overcome this high viscosity in order to shoulder their way through this dense and difficult medium. The maximum density of water occurs at 3.9 °C, which is unusual for liquids because the freezing point of water is 0 °C. The fact that water does not freeze at its maximum density allows water to exist under the ice in winter conditions and is key to sustaining life in many aquatic ecosystems. Water also has an extremely high heat capacity. It requires 1 kcal of energy to increase 1 kg of water by 1 °C. In fact, this demonstrates the importance of water to humans because many of our characteristics in physics are based on water, such as the Celsius scale of temperature and the caloric scale of energy. Because of this high specific heat, water does not change temperature very easily and remains relatively consistent over time. As a result, living in the water is actually living in a moderate thermal condition, where it is neither extremely cold nor extremely hot. In fact, fishes utilize this thermal characteristic to specialize within even narrower ranges within the typical temperatures of surface waters.

FIGURE 1-1 Schematic of penetration by different wavelengths of light into freshwater.

In addition to the characteristics above, water has several other characteristics that are important to life in the water. There is very low gas concentration in water. The atmosphere contains 21% oxygen and is relatively light and reasonable to ventilate. In contrast, water at saturation contains maximum level of 14.6 mg of oxygen for every liter of water. If we equate the two, 1 kg of air would contain 0.23 kg of oxygen, while 1 kg of water contains only 0.0000146 kg of oxygen, over 5 orders of magnitude less than the same mass of air. Clearly, this low oxygen concentration results in difficulties passing enough water across respiratory surfaces to allow animals living in water to attain high metabolic rates. This is one of the major specializations in fish - that of extracting oxygen from water at low oxygen concentration.

Water is known as the universal solvent, which means almost all materials can dissolve in water. This is both a benefit and a difficulty for aquatic life as many materials from the land and industrial processes dissolve in the water and influence physiology of fish breathing water containing that material. This universal solvent property is important in the discharge of waste as assimilation by natural ecosystems is one of the ways humans dispose of sewage. At the same time, waste dissolved in water can cause significant damage to aquatic species living in the region receiving wastes.

Finally, light is absorbed rapidly with depth in water and differentially depending on the wavelength of the light. Maximum light transmission in distilled water is approximately 100 m, and only the blue wavelengths of light penetrate to this depth. In contrast, infrared wavelengths, which include heat, only penetrate to a very shallow depth - usually less than 1 m - and there is differential distribution of wavelengths between those two extremes (Figure 1-1). These penetration depths are achieved only in very clear water; once water contains dissolved materials, there is considerably less light penetration as well as different penetration for various wavelengths. Given the oceans have a mean depth of 4,000 m, most of the ocean is below the level of light penetration, and organisms living there have difficulty using eyesight as a means of orientation.

What is a Fish?

Given the constraints placed upon fish by living in aquatic habitats, the...