Scottish Animal Welfare Commission: welfare of cleaner fish used in the Scottish salmon industry - report

Appendix IV- Biology of lumpfish

Lumpfish or lumpsucker, Cyclopterus lumpus L.

Family Cyclopteridae, Superfamily Cottoidea, Order Scorpaeniformes.

IUCN List status: Near Threatened

Most of this account is based on Davenport (1985) and Powell et al. (2018).

Description

The lumpfish has a globular body shape and gets one of its names from the modified pectoral fins which form a sucker, which is used to attach the lumpfish to rocks and other surfaces subject to forceful currents. The sucking disc is round, broad and muscular and occupies c.20% of the body length and generates as pressure of -1.0 kPa with an estimated suction efficiency of 95.8%, which is probably enhanced by mucus in addition to the sucker anatomy (Davenport and Thorsteinsson 2011). It takes 98.3-101.6 kNm^-2 force to dislodge the sucker (Davenport and Thorsteinsson 2011).

The body is roughly twice as long as it is deep. There are three lateral ridges running along each side of the body, which are marked with tubercles and which are most obvious on the dorsal ridge. The body is roughly triangular in cross section with a flattish ventral surface. The head is short with moderate lateral-facing eyes, slit-like opercula, a blunt snout with an upturned mouth with small teeth. The first dorsal fin is covered in thick skin which forms a long high crest with large compressed tubercles anteriorly and dorsally. The crest height and number of tubercles increases with age. Size, skin texture and coloration are sexually dimorphic. Males are c. 30 cm long and are red, orange or purple, eyes, and ventrum in spawning season. Females are c.42 cm long and are grey or blue-green. Lumpfish can alter their coloration in minutes to match that of algae. Light intensity and photoperiod are both important in colour matching to help avoid predators through crypsis.

Lumpfish do not have a swim bladder. Females are neutrally buoyant owing to extensive subcutaneous jelly, low-osmolarity ovarian fluid and a loos-fibred dorsal musculature which contains much water. Males are also neutrally buoyant owing to a body lipid content.

Distribution

The lumpfish’s wide distribution of 32,000 km of north Atlantic Ocean coast ranges from Disko, Greenland to Chesapeake Bay, USA (70^o – 37^o N) in the west Atlantic Ocean and from Jan Mayen, Svalbard, White and Barents Seas to southern Portugal in the east Atlantic. It is most abundant around the southern tip of Greenland, Iceland, the Faroes and the British Isles. It is a vagrant in the Mediterranean Sea.

A recent genetic study has shown that lumpfish occur in five genetically distinct stocks: West Atlantic (USA and Canada); Mid-Atlantic (Iceland); East Atlantic (Faroes, British Isles, Norway and Denmark); English Channel; Baltic Sea (Whittaker et al. 2023). Some phenotypic differences have been observed among these stocks, but it is unclear if these differences represent phenotypic plasticity or not. For example, lumpfish in the Baltic grow slowly and mature at a small size with a higher condition factor (Whittaker et al. 2023). The effective population size is lowest in the northeast Atlantic where most of the fishery occurs.

Lumpfish occupy different habitats at different life stages. Adults are solitary and are found far offshore in water up to several hundred metres deep (bentho-pelagic). They have been recorded up to 868 metres deep, but normally occur at 50-150 metres deep. However, hatchlings, juveniles and breeding adults are found in inshore waters.

Lumpfish are hardy and are found in August sea temperatures ranging from 0 to 20^oC. Although they are eurythermal, they prefer colder water. They can survive in low salinity sea water in the Baltic Sea and Hudson Bay.

Female lumpfish are semi-pelagic, and migrate annually between deep water in winter to feed to shallow inshore water to breed. Lumpfish migration may occur over long distances up to more than 1000 km, with a maximum recorded distance of 1612 kn (Kennedy et al. 2024). They may return to the same area for breeding. After spawning female disperse, travelling up to 49 km/day, while males take care of the eggs (see below). Females migrate soon after spawning. A study in Norway using acoustic trackers found that the mean time for females to disperse from the fjord was three days and the maximum was seven days (Mitamura et al. 2012).

Diet and feeding

Lumpfish feed on large planktonic organisms in surface to mid-depth waters. The intestine is about twice as long as the body and there are 36-79 pyloric caecae which aid efficient digestion. Among gut contents recorded in lumpfish are small crustaceans, such as mysids, euphausiids, amphipods, isopods and decapod larvae, and also ctenophores, polychaetes, insects, small fishes and sea grasses.

The sucker allows lumpfish to feed passively or actively. Juveniles become more active a few weeks after hatching; they cling passively to substrates when food is abundant and easy to find, but they are also active foragers if food is not abundant.

Reproduction

Lumpfish return to shallow rocky waters with high tidal surges. They are dependent on their sucker to hang on to substrates owing to their low density, large surface area and weak musculature. Neither male nor female lumpfish feed during the breeding season.

The lumpfish is a determinate batch spawner which produces a maximum of two batches of similar numbers of eggs per batch (100,000-400,000) annually within the ovary (Kennedy 2018). Eggs have been recorded being laid over a period of c. four months, but they are probably able to produce them over a longer range of time (Kennedy 2018). The eggs mature over c. eight months (Kennedy 2018). Egg masses are c.26 x c.10. x c.10 cm in size and they are usually laid among stones and in kelp (Laminaria) beds, but not on rocky substrates. The egg masses form hard clumps in the first 48 hours after spawning due to calcium and magnesium ions in the ovarian fluid. Eggs have been recorded as 2.2-3.0 mm (Davenport 1985) or 2.05-2.50 mm in diameter and larger fish produce larger eggs (Kennedy 2018); the eggs from the first egg batch.

Adult lumpfish are culled to extract eggs and milt for rearing in captivity.

Males establish territories in shallow water on rocky shores or weed-covered substrates early in the breeding season. Because females arrive asynchronously, this allows males to breed with several females. In Iceland, Norway and Newfoundland females come inshore during March-April and they spawn from early July until late August. Gravid females have been recorded from early January or early May in the English Channel. In controlled conditions it has been found that temperature controls sexual maturation, whereas photoperiod controls spawning (Puvanandran et al 2022, Mlingi et al. 2024). Manipulation of these factors in captivity can compress an annual photoperiod cycle into nine months (Puvanandran et al 2022, Mlingi et al. 2024).

External fertilisation of the eggs occurs at the nest site and lasts 5-10 seconds. As the eggs adhere into ovoid mass, they are fertilised immediately by the male, who may mould the eggs into the nest before the mass hardens. Males create funnel-like depressions in the egg mass to facilitate gaseous exchange and removal of nitrogenous waste. Older lumpfish may not recover from spawning.

Males brood the egg mass for 6-10 weeks and spend up to 99% of their time caring for and defending egg masses under favourable conditions (optimal currents and few predators) but this was reduced to 55% under unfavourable conditions (Zuyev and Rusyaev 2023). They aerate the eggs by puffing water from the mouth for up to one hour at a time and they may show signs of tiredness after long periods of aerating the egg mass (Zuyev and Rusyaev 2023). Males have also been observed fanning the egg mass with their pectoral fins and even the dorsal and caudal fins. Most aeration has been observed after the eggs have been fertilised and before the hatchlings emerge In shallow water males spit or may lie on one side with an exposed operculum in order to squirt water over the egg mass. Males may care for and defend more than one egg mass during a breeding season.

Males may remove or protect egg masses from conspecifics and predators, e.g., crabs, periwinkles, whelks, starfishes, sea urchins and fishes, such as cod and pollack (Zuyev and Rusyaev 2023). They attack fishes but may try to hide from or retreat from larger predators.

Hatchlings are 5-6 mm long and survive at first from a yolk sac and then feed on harpacticoid copepods and halactrid mites, before moving on to more active amphipods and decapod larvae. Hatchlings and juveniles, ranging 5-55 mm long, are year-round seaweed specialists, but juveniles may occur in rock pools. They feed on surface plankton after hatching and switch to algal invertebrates when larger.

Eggs develop more rapidly in warmer water. Min T 3.8 C; hatch after 40 days at 5 C and 25 days at 9.8 C. Hatch at night, 20-30 days at 10 C, ovr a period of 7-10 days.

The development of the juveniles from hatching takes 70 days in the North Sea at the start of the breeding season, but this reduces to 14 days later. Therefore eggs develop more rapidly in warmer water; at 5 ^oC they hatch after 40 days and at 9.8 ^oC they hatch after 25 days. Development in the White Sea has been recorded to take up to two months. They mainly hatch at night over a period of 7-10 days.

The suckers of hatchlings are functional from hatching and they may even stick to the male parent.

Males mature earlier, but females live longer. Spawning adults are at least four years old, but mostly range from 5-7 up to 9-10 years old. In the North Sea adults mature 1-2 years earlier. Based on otoliths in the North Sea, Iceland, Norway and Greenland, males may breed at 2-3 years old and females at 3-4 years old.

Lumpfish predators include sharks, seals and sperm whales. Males may be taken by gulls, sea eagles and otters in shallow water. Females are positively buoyant after spawning and more vulnerable to predation. The largest lumpfish weighed 9.5 kg, was 70 cm long and 14 years old.