205 
means of causing deterioration of fisheries. The most deleterious 
mode of fishing is erecting a weir across the river and permit- 
ting a trap to be fixed in it, a plan which in a very few years 
exterminates the salmon. All fixed engines are more or less 
injurious, and next to them is constant netting, one net follow- 
ing the other in rapid succession. 
Many of the lower animals are injurious to the fisheries, 
the most so being the otter, which does not restrict its meals 
to weakly or diseased fish; but it is said to do some good by 
destroying eels, which are considered to be baneful to salmon. 
Omniverous as this animal is, the contention that it principally 
lives upon cray-fishes cannot be sustained in the Severn, as here, 
these crustaceans are not found. The great black-backed gull, 
_ Larus marinus, on account of its proclivities, is known as the 
salmon-gull, and is deadly to these fishes when stranded in the 
estuary of this river. 
The fishes in the river are divisible into those which are 
present in its tidal or estuary portion, and such as are restricted 
to its fresh waters, while an intermediate class may be said to 
frequent both localities. Scientific names have been given to 
these various classes in order to signify what are their habits 
in this respect, thus some are strictly fresh water forms, as carp 
and their allies, which are restricted to the non-tidal portion of 
the river; catadromous forms are those as the common eel, 
which pass their lives in fresh waters, but when they are 
desirous of continuing their species they descend to the estuary 
and salt water; anadromous forms, as the salmon, are such as 
naturally pass most of their existence in the sea, but ascend rivers 
for the purpose of depositing their spawn, while here the eggs 
are hatched and the young reared; these latter, when suf- 
ficiently mature, descending to the ocean. Lastly, we have 
strictly marine forms, some of which may be more or less 
restricted to the shores or littoral, while others enter from the 
sea, which is mostly done in pursuit of prey. It must not 
be taken for granted that these fish are only found in the 
situations to which their habits would apparently restrict them, 
for it is evident that many marine rapacious forms will follow 
P2 
