The growth rate in B. pholis has been studied mainly from the zones on the otolith. One wide opaque zone and one narrower transparent zone are laid down during the course of a year. The transparent zones are formed in spring, from March to May, following a period of minimum sea temperatures, reduced food intake, maximum gonad development and no observed growth in length of the fish. The opaque zones are formed during the rest of the year, mainly during the summer period, to which the whole of the growth in length is confined.
Seasonal size frequency distribution curves show distinct peaks which evidently correspond to two or three year classes, because they fall in close agreement with the lengths of these classes determined from direct otolith readings and from back calculations.
Otoliths show slight negative heterogonic growth, which was allowed for in the back calculations. Both sexes show a similar growth rate and attain a similar longevity.
Growth in length seems to occur mainly during summer in older age groups, but in the first year group it seems to continue at a reduced rate during autumn and winter. Growth is rapid during the first two years. It gradually decreases in subsequent years. This causes the older age groups to overlap considerably in size.
A study of the seasonal changes in the gonads of B. pholis, based on an arbitrary classification of maturity, showed that both sexes mature when nearly two years old and begin to spawn when about 8 cm. in length. The males ripen earlier and remain ripe for a longer period than the females. Each individual spawns successively during the breeding season, which lasts from April to August. There is a quick recovery of the gonads in both sexes after spawning is over. Ripe fish occur throughout spring and summer but no spent females were seen until July. Eggs were found in abundance on the shore from mid-April to mid-August, with a maximum in June.
Seasonal changes in gonad weight are less marked in males than in females. The ovaries reach their maximum weight during May, in preparation for maximum spawning, which occurs in June.
In both sexes there is a regular seasonal cycle in the “condition factor” which cannot be explained as being due to seasonal changes in the gonad weight alone. Alternating periods of growth in weight and growth in length seem to account for most of the seasonal rise and fall in the condition factor. There is a temporary fall in February and March, which is probably due to net loss of reserves on account of reduced feeding at the coldest season. The mean K values obtained for each length group, from the pooled data of all seasons, gave no indication of the size at first maturity, but some evidence of this was obtained by separate treatment of the samples from the months when spent fish were abundant.
The size frequency distributions of oocytes in ripening and ripe ovaries show multiple groups, which are successively ripened and shed during the breeding season.
The food of Blennius pholis was investigated by the examination of the gut contents. Qualitative estimation of the various categories of food in the gut revealed that B. pholis feeds on a wide variety of common shore invertebrates and plants. The bulk of food is, however, made up of Crustacea, Polychaeta and Mollusca. Barnacles form the chief food throughout the year. The composition of food varies slightly in certain seasons, presumably being regulated by the effect of winter conditions and nesting habits on the searching activity of the fish.
The quantity of food consumed by the larger fish also differs in different seasons: the feeding intensity seems to be related to the seasonal changes in the sea temperatures and breeding activities. Maximum feeding occurs during July, August and September and continues until November.
The food of the O group fish is similar in nature to those of the older age groups. Barnacles, smaller Crustacea and other arthropods form an even higher proportion of the food. Organisms such as crabs, hydrozoans, bryozoans and algae, which occur throughout the year in the guts of the older fish, are not readily eaten by the smaller fish.