1. Which fish called shishamo really comes onto the beach to spawn?
According to materials from the Hokkaido Research Organization, true shishamo live only along the Pacific coast of Hokkaido and swim upstream in rivers to spawn [1]. The karafuto-shishamo seen often in supermarkets is a different species. The Department of Fisheries and Oceans Canada (DFO) explains that capelin come rolling onto beaches with sand or fine gravel during spawning season, where males and females release sperm and eggs [2]. The eggs stick to the grains of gravel. Imagining fish writhing their bodies at the water’s edge to spawn is quite dramatic.
When do they spawn? According to DFO, spawning happens mostly at night or during high tide, and it is "thought" that winds blowing toward land and the phases of the moon also relate to the timing [2]. The reasons are not written in detail, so we treat it as something researchers think may be related. But are beaches the only place capelin spawn?
2. How do capelin choose between spawning on the beach and on the offshore seafloor?
In surveys off Newfoundland in eastern Canada, capelin were found spawning in two places: on the beach and on cold seafloors at depths of about 15 to 40 m (about 50 to 130 feet) [3]. Spawning occurred within a range of about 2 to 12°C (about 36 to 54°F). It was reported that they did not spawn in places below 2°C or where temperatures stayed above 12°C. This is a span of 10 degrees, meaning "not too cold and not too warm."
This range relates to how they choose their location. In early July, the offshore seafloor is still too cold, so they spawn on the beach. When the beach exceeds 12°C and becomes warm, spawning on the cold offshore seafloor increases [3]. DFO also explains that spawning sites are chosen mainly by water temperature, and when beaches exceed 12°C, spawning on the offshore seafloor increases. Offshore sites are described as being at depths less than 40 m (about 130 feet) on sandy or gravelly bottoms, which are colder and saltier than the beach [2]. In this study area, it seems that the use of beach versus offshore sites was linked to water temperature. However, asserting that only water temperature decides it is going too far, as other conditions may also be involved.
3. Do capelin eggs on the beach and on the seafloor hatch at different times?
DFO explanations state the timing for larvae to emerge from eggs. Beach eggs hatch 2 to 3 weeks after spawning, while offshore seafloor eggs take more than 4 weeks [2]. Since 2 to 3 weeks is 14 to 21 days and 4 weeks is 28 days, offshore hatching is at least about one week slower. Generally, the lower the water temperature, the slower egg development proceeds. However, the materials read here do not directly explain the reason for this difference in days, so it is safe to keep the fact that "offshore eggs hatch later" separate from the guess that "water temperature causes it." Spawning on the beach could be seen as a way to start life sooner because eggs hatch quickly, but this is also just our imagination, not a reason stated in the materials.
4. Do capelin in northern seas like the Barents Sea spawn mostly on the seafloor?
According to the Norwegian Institute of Marine Research (IMR), capelin in the Barents Sea head toward the coast in January and February, spawning along the coasts of Troms and Finnmark in northern Norway and the Kola Peninsula in Russia [4]. They spawn on seafloors at depths of 20 to 60 m (about 65 to 200 feet). After spawning, they soon die. Most reach spawning age at 3 to 4 years, and it is rare for them to live beyond 5 years [4].
Icelandic capelin are said to begin their spawning journey in December or January, moving clockwise along the coast from north to southeast. Spawning occurs mainly in late March to early April on shallow coastal seafloors. Survival rates after spawning are very low and can practically be treated as zero, with many spawning at age 3 [5]. The choice between beach or seafloor changes depending on location and season. Beaches are used in warm summer periods, while seafloors are used in colder periods or deeper areas. In the Barents Sea and Iceland, seafloors seem to be the main site.
5. Do capelin schools gathering near shore become food for other marine animals?
Capelin are considered an important "forage fish" in the marine food web. DFO also calls this fish a key forage species [2]. According to IMR, cod eat large amounts of them in the Barents Sea, along with harp seals, some whales, and seabirds [4]. The spawning season when schools gather on shore is thought to be an opportunity for these predators because food increases in one place. However, this is an inference drawn from the text; no data showing how many predators actually gather during spawning season was read here. Why do they spawn in schools? The advantages of spawning in groups are not clear here. We leave it as "something unknown."
6. How can you investigate the shishamo sold in a supermarket at home?
If you go to a supermarket with an adult, try reading the label on the "shishamo" package. Check if the fish name (it might say "karafuto-shishamo") or the sea where it was caught is listed. You might find out if it is the fish discussed in this article. Look up Newfoundland, the Barents Sea, and Iceland in an atlas. They are all cold northern seas. Imagining fish gathering on beaches or seafloors there makes the talk about water temperature feel more real. If you want to know more, compare these three seas on a map and find the northernmost one. Guessing how spawning places and seasons might change in colder seas is a fun observation.
A remaining question for further thought is: How does the choice of spawning place move if beach water temperatures change?