Taxonomy

Rarity

Abundance rates how many are in suitable habitat. Find difficulty rates how hard it is to locate one on a daylight snorkel or freedive through 50 ft. A higher find score means a harder search. The rarity category also reflects the kind of encounter; it is not an average of the 2 scores.

Seasonality

Location

Topsmelt range from the Gulf of California north to a point about 4 miles west of Sooke Harbour on Vancouver Island, British Columbia, and the range includes Guadalupe Island off Baja California. That is a long coastline for a fish under 15 inches. They live in the surface water of bays, sloughs, estuaries and kelp beds, over mud and over rock, and they concentrate in the top 15 feet. The common name is a description of the habit. Depth records reach about 85 feet, but a topsmelt spends its life near the ceiling.

Size varies with latitude in a way worth knowing. The largest fish are found at the northern end of the range, and the biggest topsmelt ever recorded measured 14.5 inches, against 17.5 inches for the largest jacksmelt. Most of what a southern California diver sees is under 10 inches. Growth is quick and then stops mattering. A topsmelt puts on 2.5 to 4 inches in its first year and another 2 inches in its second, by which time most are mature, and the oldest fish measured have been 7 to 8 years.

Spawning happens from the middle of May through early July, in shallow water, with a female accompanied by several males. The eggs go onto eelgrass blades and low algae, held there and to each other by sticky filaments. After hatching, the young move into the surface canopy of the kelp forest, where topsmelt larvae and juveniles occur in enormous numbers alongside young jacksmelt. The bright clouds of small silver fish at the top of a kelp bed in early summer are mostly these 2 species.

Deep Dive

Topsmelt are named for a habit and misnamed for a family. The first half is accurate: they swim at the top. The second half is not, because a topsmelt is not a smelt at all. True smelt are closer relatives of trout, carry a small fleshy adipose fin between the dorsal fin and the tail, have a single dorsal fin, and have pectoral fins set low on the body. Topsmelt have no adipose fin, 2 dorsal fins, and pectoral fins set high. Their nearest relatives are the mullet and the barracuda. California ocean water holds 3 silversides, and the other 2 are the jacksmelt and the California grunion.

Telling a topsmelt from a jacksmelt takes 2 characters. On a topsmelt, the anal fin, the unpaired fin on the underside behind the vent, starts directly below the back end of the first dorsal fin. On a jacksmelt it starts between the 2 dorsal fins. The teeth differ as well: topsmelt teeth are forked and sit in a single row along the jaw, while jacksmelt teeth are unforked and set in several bands. The color is the same on both, green over blue-green above, silver on the sides, with the blue stripe down the middle that gives the whole family its name.

The species has a history of being split apart and put back together. Topsmelt from different bays and different habitats look different enough that biologists once treated them as separate subspecies. That has not held. Topsmelt has been recognized as a single species for more than 25 years, and a 2004 study by Kathleen O'Reilly and Michael Horn measured the shape variation directly and found it to be variation within a single species rather than evidence of several.

What the fish does with its gut is the reason that variation exists, and it is the most interesting thing about the animal. Silversides have no stomach. Food goes from the throat straight into the intestine with no acid chamber between them. In 2006 a team led by Horn published a study in Marine Biology comparing the digestive systems of topsmelt, jacksmelt and grunion, and found that the 3 species had taken that same stomachless plumbing in 3 directions. Grunion eat only animals. Jacksmelt are mostly carnivores with some plant material. Topsmelt do both, and which one depends on where the individual fish lives. Topsmelt in estuaries eat mostly algae. Topsmelt in kelp forests eat mostly animals. The study measured the difference inside the fish, not just in the stomach contents: estuary topsmelt had the longest guts, the largest surface area in the intestinal lining, and the highest activity of the enzymes that break down starch. The same species, a few miles apart, is built differently to match what it is eating.

The other extreme trait is salt. Topsmelt handle a range of salinity that almost nothing else on this coast can match, from fresh water at 1 end to water far saltier than the sea at the other. Ordinary seawater runs about 34 parts per thousand. When the State Water Resources Control Board needed to know how California test species would react to the brine that desalination plants discharge, a University of California, Davis team ran the numbers, and larval topsmelt survived to about 60 parts per thousand before half the test animals died. That put topsmelt among the most salt-tolerant of the 9 organisms tested, alongside mysid shrimp, which are small shrimp-like crustaceans, and giant kelp.

That tolerance is also why the fish shows up in laboratories. Topsmelt is 1 of the 9 standard test species written into the California Ocean Plan, which means the state uses it to decide whether a discharge is toxic to coastal water. Larval topsmelt survival and growth are the measurement. A fish that will keep living across a wide range of temperature and salinity makes a clean test animal, because a result can be attributed to the contaminant rather than to the conditions of the test.

For everything else in the water, topsmelt are food. The Department of Fish and Wildlife lists them as an important forage species for marine birds, fish and mammals, and other accounts name sea lions and seals specifically. A school of topsmelt in the kelp canopy is not the interesting animal on a dive. It is the reason the interesting animal is there.

Where to Find Them

Researched habitat and range candidates

These locations have suitable habitat within the species range. An assignment is a possible encounter, not a confirmed sighting at every beach. Season, depth, and local conditions still matter. Historical habitat without present-day evidence is excluded.