Guest post: Shemaiah Kentish, George Mason University
An alevin with a orange yolk-sac attached (photo credit: fishbio.com)
An alevin is a newly hatched salmon or trout that still has its yolk-sac attached. The yolk-sac provides the alevin nourishment for the first 2-3 weeks after hatching. Typically, at this life stage, salmon or trout will stay in the gravel but they might be pushed into open water by a strong current, making them more susceptible to predation.
‘O’opu nakea (Awaous stamineus) is an important cultural resource for Indigenous Hawaiians. All five Hawaiian gobies exhibit amphidromous life cycles.
An amphidromous fish is a type of diadromous fish which migrates between fresh and saltwater. Unlike anadromous and catadromous fish, which migrate explicitly for the purposes of breeding, amphidromous fish migrate for other purposes. A typical cycle of amphidromy includes eggs hatching in freshwater or estuaries, larvae drifting out into the open ocean to feed and grow, juveniles returning to freshwater to feed and grow, and adults reproducing in freshwater. Gobies, such as those found on volcanic islands in the Pacific, exhibit an amphidromous life cycle.
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And, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!
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.
Coho salmon are one of many salmon species, their smolt lifestage is pictured here.
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?
George S. Myers defined the term diadromous to refer to fish that migrate between fresh and salt water.
Diadromous fish migrate between fresh and salt water. This term, coined by George S. Myers in 1949, refers to both anadromous and catadromous fishes. Diadromous life history strategies have evolved in numerous fish taxa. The complex life cycles do require complex osmoregulatory adaptation but the advantages of the given environments for specific life stages of the fish outweigh the costs.
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Pacific salmons are well-known examples of anadromy. They live most of their lives at sea but spawn in fresh water
An anadromous fish hatches and spawns in fresh water but spends most of its life in the salt water. This dual life cycle allows the younger fish to grow and feed in the less perilous fresh water habitats (fewer predators in the smaller systems) and the older, larger fish to grow and feed in the marine systems, where there is a greater prey base. Migrating between fresh and salt water requires complex osmoregulatory adaptations. Some species, such as Pacific salmons, only transition between fresh and salt water twice, migrating to salt water early in life and then migrating back to freshwater to spawn once and die (a semelparous reproductive strategy). Other species, such as Striped Bass (Morone saxatilis) will migrate between fresh and salt water every year to spawn (an iteroparous reproductive strategy).
For more information, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!
Striped bass are anadromous fish that migrate between fresh and salt water every year to spawn