Effects on Wild Young Salmon of Spraying DDT over New Brunswick Forests

Abstract

When DDT-in-oil was sprayed over large tracts of New Brunswick forest some of the spray got into drainage systems lying in the spray zones. As a result, many young salmon were killed and stocks were endangered.Most New Brunswick salmon spend 3 years in rivers before migrating to sea as smolts. The young fish can conveniently be divided into three size-groups, which are roughly comparable to each year of age. The groups are: underyearlings, small parr (over 1 year old but 10 cm and under from tip of snout to tip of extended tail fin), and large parr.Wild young salmon in unsprayed and sprayed streams were assessed for numbers in each size-group, abundance being expressed as population index = average number per 100 yards2 of stream bottom.DDT at[Formula: see text]. After such spraying young salmon of all size-groups were found in noticeably low numbers as compared with unsprayed situations. Underyearlings were only 2–10% as abundant (index reduced from 24 in places not affected by spray to less than 1 in most sprayed places), small parr only 30% (index reduced from 20 to 6), and large parr only 50% (index reduced from 12 to 6) as abundant.DDT at[Formula: see text]. After spraying at this dosage underyearlings were only about 50% as abundant as with no spraying; small parr were about 80% as abundant and large parr were affected hardly at all.DDT at[Formula: see text]applied twice. This was followed by low numbers of underyearlings, similar to those observed with a single application at [Formula: see text]. No equivalent data for parr are available but other information indicates effects similar to a single spraying with [Formula: see text].Downstream transport of the DDT-in-oil mixture was an important factor in extending the, harmful effects 30 or more miles below spray zones.Delayed mortality followed 4 to 6 months after the early mortality described above. It was associated with the onset of winter cold. It may have removed a large proportion of parr which survived early mortality but quantitative measurements of this factor were not made on wild populations.Reductions in numbers of young salmon were followed by declines in numbers of adults taken in fisheries and returning as spawners.With no spraying in subsequent years, promptness of recovery of young salmon populations depended primarily on abundance of parent stock.