Production of a protein feedstock (I) comprises: (a) comminuting fishery waste (II); (b) adjusting to an acidic pH to encourage microbial digestion; (c) microbially digesting at 25-38 C; (d) screening to give a liquid phase, and (e) ageing the liquid phase. Also claimed are: (1) (I) obtained by the above process; (2) a fertiliser composition comprising (I) emulsified with an oily phase (preferably a fish or vegetable oil, especially a master composition comprising the oil phase and an emulsifier) forming 0.5-5 vol.% of the composition; (3) production of an oil master composition for use in oil-in-water emulsion fertiliser compositions, by treating a vegetable oil with sufficient surfactant (preferably nonionic) to emulsify the oil in an aqueous phase, removing precipitated gum and adding an antioxidant (preferably dl--tocopherol acetate); (4) comminuter apparatus including a housing having an inlet opening including a shearing edge; a shearing blade rotatable in the housing, having a leading edge extending outward from a hub portion of the blade having an outer edge portion curving in the direction or rotation and adapted to cooperate with the shearing edge, and mincing blades mounted for co-rotation with the shearing blade, and (5) a nematocidal soil treatment involving applying (I) to the soil.    (II) is one or more of filleting wastes (fish frames), bycatch or waste whole fish, crustaceans and the like, and is comminuted to an average particle size of 6-25 mm. Comminution is carried out such that the temperature of the waste does not exceed 35 C, and the moisture content of the comminuted (II) is 50-99 wt.% (adjusted if necessary). The digesting microorganism is selected from acidophilic bacteria which are obligate or facultative anaerobes, especially Lactobacillus acidophilus. Digestion is at pH 4.5-5.0, preferably in presence of an antimicrobial agent which permits survival of acidophilic anaerobes (preferably sorbic acid or sorbate, used at 0.001-0.2 wt.% measured as sorbic acid). In the nematocidal method, (I) is applied to the oil at 60 l/ha; or alternatively (I) is compounded with an emulsifiable oil and an active culture of a microorganism (preferably the dormant culture present in (I)) and applied at ca. 40 l/ha (as protein).    USE -   (I) is useful (optionally in combination with an oil phase) as a fertiliser, feed supplement for livestock or aquaculture or nematocide. (II) (e.g. waste from the Australian orange roughy, which has a high waste ratio) can be converted into a source of nitrogen, phosphorus, potassium and trace minerals required by plants for healthy growth. (I) is also useful in aquaculture, especially fish farming, as a feed supplement and as a protein supplement in stock feeds, e.g. as a substitute for kelp in preventing or treating iodine or selenium deficiency in horses, cattle and other grazing animals.    ADVANTAGE -   (I) is obtained by a reliable and efficient method. Use of (I) as a fertiliser, in combination with an oil phase, markedly reduces aquatic plant biomass production due to leaching of nutrients into water courses. The comminuting apparatus eliminates excessive heating of (II). A combination of (I) with an emulsifiable oil and an active microorganism culture has synergistic nematocidal activity.