Cod: Gadus morha (L.)

Phylum: Chordata        Class: Osteichthyes        Order: Gadiforms        Family: Gadidae

Species description

Cod are a typical fish shape with an elongated body however, quite uniquely they have; three dorsal fins, two anal fins, the upper jaw overhangs the lower jaw and the lower jaw possess a barbel [1]. The lateral line is prominently curved and whitish in colour, above this is mottled brown in colour and below is paler, almost white [1]. Cod often form large schools during the day [6].

BIOLOGY and ECOLOGY

Reproductive behaviour

Cod spawn in the southern North Sea and eastern English Channel from January through to April, peaking in late February [8]. The area of the Channel between Beachy Head and Dungeness has been identified as one of the U.K.’s main cod spawning areas [8]. Cod exhibit pre-spawning courtship behaviour. This begins with a fin display and grunting by the males, the pair then move towards the surface and swim in circles with the male inverted below the female whilst spawning occurs [2]. The eggs are 1 - 2mm in size and are buoyant, thus planktonic. Hatching occurs at approximately 4mm; growth rate is temperature dependant but usually hatching occurs between 8 - 23 days [2]. The larvae then drift to nursery areas and at a size of approximately 20mm, 60 - 75 days old they become demersal. The juveniles are known as ‘codling’ and remain on ‘rough’ ground for 1.5 - 2 years. Maturity occurs at 4 - 5 years of age, 68 - 78cm in length.

Migratory behaviour

Within the Sussex IFCA district cod begin to migrate inshore to their spawning grounds during autumn and spawning occurs during winter and spring [8]. They then move to feeding areas, which may be associated with herring abundance [2]. A recent tagging study by CEFAS indicated that cod in ICES square VIId (which encompasses the Sussex IFCA district) do not have a consistent pattern of large seasonal movement. Some moved out of the English Channel into the North Sea but many were recaptured close to their release position [7]. Special electronic tags were used to monitor the fish’s behaviour. The results found their distribution to be quite closely matched both seasonally and ontogenetically, indicating no obvious pattern of large redistribution of the cod in VIId [7]. In comparison the cod in the adjacent ICES area IVc migrate northwards and eastwards using tidal streams during spring [7]. It is suggested that these behavioral differences limit the mixing of cod from these two areas during feeding and spawning seasons [7]. In addition, it was noted that during autumn and winter the cod were a) more likely to be recaptured closer to their release position and b) there was a greater proportion of overlap between the ‘juvenile’ range and the ‘adult’ range [7].

Habitat

Cod spend most of their adult life close to the sea bed and can be found from the shoreline to 600m deep [3].

Predators and prey

Cod are voracious predators feeding on a large variety of zoo benthic invertebrates and fish, including smaller cod. Most of their feeding activity takes place at dawn and dusk [6]. The main predators of young cod include larger cod, squid and pollock, while a common predator of larger cod are marine mammals, notably seals [6].

THE SUSSEX FISHERY

Fishing methods

Fishing activity

Cod begin to migrate inshore into the Sussex IFCA district during autumn and move further offshore again in late winter/early spring, during this time inshore breeding occurs and catch rates are highest. Due to strict TAC quota regulations in recent years, cod is rarely a target species and is more commonly landed as a bycatch; specially in the use of fixed nets and stern trawls whose target species is most commonly flatfish. A small proportion of cod will also be taken in keddle nets in the vicinity of Rye bay, these nets are set in the evening and recovered the following morning with catches being at their best following a spell of unsettled weather. There appears to be no difference in the catch rate of cod between day and night. It is important to note that the majority of the cod caught and landed within the district are codling; between 1 to 3 years of age and thus are sexually immature.

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