The Cyclothone microdon is a small mesopelagic fish that is not particularly sought after for sport fishing due to its small size and discreet nature. However, it is fascinating for its role in the marine ecosystem as prey for many predators. In Quebec, its presence is noted mainly in the deep, cold waters of the Gulf of St. Lawrence. This fish is a typical example of the species that make up the bathypelagic community, living at depths where daylight is absent. Although little known to amateur fishermen, it attracts scientists' attention for its adaptations to life in deep waters and its abundance in the open sea.

The Cyclothone microdon is a small fish, typically measuring between 5 and 15 centimeters. Its body is elongated and slightly laterally compressed, giving it a slender silhouette. Its skin is brownish-black, almost translucent, with photophores along its body, allowing it to produce light, an essential asset in the dark depths. The mouth is relatively large compared to the fish's size, equipped with small fine teeth, hence its name. The fins are small and discreet, adapted for swimming in deep waters. The scales are fine and not very visible, often lost during capture. Young fish resemble adults closely, although their bodies are generally lighter. There is no marked sexual dimorphism, but females may be slightly larger during the spawning period.

In Quebec, the Cyclothone microdon is mainly found in the Gulf of St. Lawrence, where the deep, cold conditions are ideal for its survival. It is rarely observed near the coast due to its preference for deep waters. Although widely distributed in the world's oceans, its presence in Quebec waters is limited to areas beyond the continental shelf, where it forms significant schools. Its density can vary, but it is generally more abundant in regions where ocean currents bring nutrient-rich waters.

The Cyclothone microdon primarily inhabits mesopelagic and bathypelagic zones, typically between 200 and 1000 meters deep. It prefers cold, dark waters where it can escape many predators. These fish are often associated with areas where ocean currents nourish the water with nutrients, promoting a rich food chain. In the Gulf of St. Lawrence, they are often found in open waters, away from coastal structures, and their presence is influenced by oceanic conditions such as temperature and salinity.

The life cycle of the Cyclothone microdon begins with the laying of eggs, which float freely in the water column. After hatching, the larvae develop rapidly, benefiting from the nutrient-rich waters teeming with plankton. Sexual maturity is reached in one to two years, depending on environmental conditions. These fish have a relatively short lifespan of two to five years, which is typical of species living in deep waters and harsh living conditions. Spring → Spawning and larval development → Summer → Growth of juveniles → Autumn → Maturity and reproduction → Winter → Life in depth and survival.

The Cyclothone microdon is a gregarious species, often forming large schools to protect itself from predators. It is not territorial and moves with currents and food availability. Sensitive to changes in light, it migrates vertically, rising at night to shallower waters to feed and descending during the day to avoid visual predators. This behavior is influenced by weather, winds, and atmospheric pressures that affect ocean currents.

The diet of the Cyclothone microdon mainly consists of zooplankton, small crustaceans, and fish larvae. Young fish primarily feed on plankton, while adults also capture larger prey such as small fish and shrimp. The diet varies with the seasons, increasing in summer when plankton is more abundant. In winter, their feeding activity decreases, but they continue to opportunistically feed.

The Cyclothone microdon is not subject to traditional fishing seasons, as it lives at depths inaccessible to sport fishing. However, its feeding activity follows seasonal cycles, with increased activity in spring and summer. The best times to observe its behavior are at night during its vertical migration. Weather conditions have little effect on this species, but stable atmospheric pressures and moderate winds may encourage its migration to shallower waters to feed.

For those wishing to study the Cyclothone microdon, it is recommended to use lures or baits that mimic planktonic organisms. In clear waters, natural and translucent colors are effective, while in murkier waters, brighter colors may be necessary. The depth at which one works is crucial, as these fish typically live hundreds of meters below the surface.

For the scientific capture of the Cyclothone microdon, specialized equipment includes plankton nets and deep-sea sampling devices. A traditional fishing rod is not suitable for this species due to its small size and deep habitat. Researchers use fine lines to minimize detection and measuring devices to record environmental data.

Handling the Cyclothone microdon requires delicacy due to its fragility. When captured, avoid handling it directly to prevent damaging its delicate skin. For release, ensure that the environment is similar to its natural habitat. Taking photos should be done quickly to minimize stress on the fish.

Predators of the Cyclothone microdon include larger fish such as sharks and tunas, as well as seabirds like gulls during its nocturnal vertical migrations. These predators take advantage of its small size and abundance to feed efficiently.

The Cyclothone microdon is currently not subject to any specific regulations in Quebec, due to its low commercial and sport value. However, as a key species in the marine food chain, it is important to monitor populations to understand the impacts of environmental changes. Release is not a common practice for this species, except in a scientific context.