Escapement is a term used in salmon management to mean, quite simply, how many salmon are able to “escape” premature death and complete their full life cycle. These escapees have run the gauntlet of natural mortality, recreational fishing, commercial fishing, and other life-threatening episodes for the opportunity to spawn in freshwater. Escapement is estimated by a number of different methods including fish counts (e.g., at a weir or dam viewing window) and carcass surveys (i.e., counting the dead salmon on spawning grounds). For salmon managers, the amount of escapement for each salmon run informs population estimates and appropriate fishing quotas and other management strategies for future fishing seasons.
A fish species is considered endemic to a location if that is the only place it occurs naturally. It may have evolved in that region or over time has become so adapted to that specific environment that it cannot survive elsewhere. For example, Devil’s Hole Pupfish (Cyprinodon diabolis) is only found in Devil’s Hole, Nevada. If a species is naturally located in multiple locations, it is considered a native species, but is not an endemic one. Many endemic species are T&E species, threatened with extinction due to loss or modification of their habitat by agriculture, urbanization, or other human land uses.
Elasmobranchs, including sharks, rays, and sawfishes, belong to the taxonomic subclass of cartilaginous fish Elasmobranchii. Like most chondrichthyes, they have exposed gills, no swim bladder, internal fertilization, and placoid denticles. They differ from the other subclass, chimaera (subclass: Holocephali), in that they have rigid dorsal fins, placoid denticles cover most of their bodies, and they usually have spiracles (modified gill slits directly behind the eye).
In contrast to stenotherms, eurythermic fish can function at a wide range of water temperatures. They are often, but not necessarily, ectotherms. Desert Pupfish (Cyprinodon macularius), for example, can function in ambient temperatures ranging from 4 to 45 degrees Celsius. This thermoregulatory strategy requires that organs, enzymes, and metabolic processes can operate at varying environmental temperatures.
Unlike endotherms which can metabolically control their own body temperature, ectotherms rely upon environmental temperatures for thermoregulation. Most fish are ectotherms. Ectothermy can be metabolically more efficient than endothermy because organisms do not have to expend any energy to self-regulate their body temperatures. However, they are at the mercy of their environment more than endotherms because they use ambient water temperature to control their body temperature.
Poikilothermic fish have no control over their body temperature whatsoever. Their core body temperature conforms entirely to ambient temperature and can fluctuate widely. This means that their organs and enzymes need to be capable of functioning at a range of temperatures. As physiological processes have often evolved to operate most efficiently at certain temperatures, ectothermic fish have evolved multiple strategies to maintain optimal thermal habitat. Stenotherms live within narrow environmental temperatures in contrast with eurytherms which can live in a wide range of environmental temperatures.
Unlike ectotherms, which rely upon environmental temperatures, endotherms are able to metabolically control their body temperature. This thermoregulatory strategy is rare among fish but is present in tunas and some sharks, including the Great White Shark (Carcharodon carcharias) and Shortfin Mako Shark (Isurus oxyrinchus). Endothermic tunas and sharks use a network of capillaries in their swimming muscles, the Rete mirabile, as a heat exchanger. Through counter-current exchange, the heat produced through muscle activity is transported by the blood. Through this metabolic process, sharks, for example, can maintain a body temperature of 5 – 14°C above ambient water temperature. This process is an evolutionary advantage for these long distance, migratory fish, allowing them to travel extensive distances and dive deep while maintaining body temperature, conserving energy, and avoiding thermal shock from changes in water temperature.
Electrofishing is a survey tool used to estimate species composition, fish abundance, and fish density in freshwater systems using an electrical current. An electroshocker creates a flow of current between an anode (i.e., ring on a backpack models) and cathode (i.e., rat tail on backpack models) that stuns fish that cross the electric field. Stunning the fish makes them easier to capture and assess for research and monitoring purposes. Backpack electroshockers are generally used for smaller, wadeable waterbodies; boat or raft mounted electroshockers are generally used for larger waterbodies. While electrofishing can be a very effective technique for freshwater, electricity has poor conductivity in salt water.