Natural Mortality

Abstract

As we mentioned in §3.2, the theoretical treatment of natural mortality in fish populations presents certain difficulties, these being due largely to the scarcity of information relating to the causes of natural death in adult marine fish. Predation is perhaps the most general ‘external’ cause, and the evidence has been reviewed by Errington (1946). The majority of recorded instances of predation refer to lake or river fish, but even here, direct evidence and measurement of the effect on the prey population seem relatively rare. Perhaps the most striking example in commercial fisheries is that of the predatory activity of the sea-lamprey (Petromyzon marinus) on certain fishes of the Great Lakes, discussed by Applegate (1951) (see also §9.4.3.2.4). Again, according to White (1939)-quoted by Huntsman (1948)-fish-eating birds are sometimes a major cause of mortality in salmon, and their removal results in an increase in the salmon population. Among other causes may be mentioned the low temperatures in the North Sea in 1929 (Lumby and Atkinson, 1929) and again in 1947 (Simpson, 1953), which was responsible for the death of large numbers of demersal fish,* and the poisonous effect of the dinoflagellate Gymnodinium brevis in the waters off the west coast of Florida (Gunter, 1949). It would seem that the latter is spasmodic in its occurrence, but it is possible that low winter temperatures in the North Sea are a regular component of the natural hazard in certain species, though to a much lesser extent than in the abnormally severe years mentioned above.