Guest post by Emily Argo
Each year adult salmon migrate upstream to spawn (lay eggs) putting into motion their offsprings’ journey from hatching to making their way back to the ocean as juveniles. In addition to being a spectacular event to witness, these migrations also play a central role in ecosystem health. Over 100 different species benefit from the nutrients that the salmon provide! In order for a juvenile salmon to successfully complete their journey to the ocean and continue providing benefits to marine and freshwater ecosystems when they return to as adults, the juveniles must undergo a series of changes through a process called smoltification.
Smoltification occurs in anadromous species prior to entering saltwater. For example, as salmon juveniles move from their freshwater nursery habitats to the ocean, they transition from what scientists call parr to a smolt lifestage. During this transition, they change color, losing their parr bars (dark, vertical bars) and defined countershading, making them appear silvery. Their bodies also change to be more streamlined and buoyant. Even their attitudes change! While parr are typically highly territorial in shallow water, this aggression declines as the smolts move into deeper water and form shoals (large groups). During smoltification salmon smolts get a good whiff of the water to imprint on the odor of their natal stream (stream where they were born) in order to identify it during their return spawning migration as adults. Finally, how the salmon breathe (take oxygen out of the water with their gills) also changes as they go from freshwater to saltwater.
Smoltification does not begin at a specific time in all salmon species, but varies depending on temperature, latitude, size, rate of growth, age, and feeding opportunities or a combination of these factors. Since the changes that take place during smoltification are not necessarily beneficial for remaining in freshwater, the changes will reverse (desmoltification) for fish who are unable to migrate to saltwater. This is sometimes the case when there is a barrier to migration, which can cause fish to remain in freshwater throughout their lifecycle. Luckily for these individuals, salmon are equipped to survive in freshwater as long as they need to, but the inability to migrate will limit their ability to provide important nutrients to these freshwater ecosystems on their return migration as adults.
How many species of salmon can you name?
A scute is an external bony plate on the surface of a fish. Scutes serve a protective function, acting as a body armor for fish against environmental abrasions and even predation. In some fishes, such as Alewife (Alosa pseudoharengus), scutes are a row of scales modified into sharp, protective plates. In other fishes, like Shovelnose Sturgeon (Scaphirhynchus platorynchus), scutes serve a homologous function but are derived from ossified deposits in the dermis (essentially bone over skin).
The swim bladder is an air filled organ used by some fish to maintain buoyancy at a desired depth and produce or hear sound.
- Physostomous swim bladders are directly connected to the gastrointestinal tract so that fish with these swim bladders, such as herrings, must “gulp” air to inflate their swim bladder and “burb” or “fart” air to deflate them.
- Physoclistous swim bladders are not connected to the digestive tract so that fish with these swim bladders must diffuse gas from the blood to fill and collapse them.
For many fish, the swim bladder has the additional role of transferring sound waves to the auditory system. And in some fish, such as drums and croakers, the swim bladder is used to make sounds and communicate with one another and other fish.
In contrast to eurytherms, senothermic fish can only function in a narrow range of water temperatures. Brook Trout (Salvelinus fontinalis), for example, function optimally approximately between 13 and 18 degrees Celsius. This thermoregulatory strategy requires that organs, enzymes, and metabolic processes operate in a small temperature band and makes these fish particularly vulnerable to environmental changes.
Schreckstoff, German for “scary stuff,” is a chemical compound, glycosaminoglycan chondroitin, that some fish species release when they are injured. This “take-one -for-the-team” signal lets their conspecifics know that there is danger in the area. But more than just an alarm signal, Schreckstoff also serves as an immune response for the injured individual, warding off parasites and pathogens such as molds, trematodes, and solar radiation. Schreckstoff is utilized by fish superorder, Ostariophysi, such as minnows, catfishes, and characiformes (including piranha and tetras), and has also been documented in other species, such as salmonids.
FISH MATING STRATEGIES
In honor of Valentines Day, please enjoy the first series of post for this blog post on fish mating strategies! Check back weekly for new additions to the “Fish-ionary.”
- Promiscuous: A mating system where both sexes have multiple partners during the breeding system. In fishes, this is the most common mating system. Breeders make little or no mate choice and spawn with multiple partners, either sequentially or at the same time….read more.
- Polygamous: A mating system in which an individual of one sex has multiple partners during the breeding system but individuals of the opposite sex have only one partner…. read more.
- Polygyny: A polygamous mating system where a male has multiple female partners during the breeding season. This is the more common form of polygamy….read more.
- Polyandry: A polygamous mating system where a female has multiple male partners during the breeding season….read more.
- Monogamy: A mating system where partners live and exclusively mate with only each other….read more.