The annual reproductive cycle of pike, Esox lucius L., in two Scottish lakes

Abstract

Histological changes in the ovaries of pike from two shallow, adjacent, Scottish lakes are described and related to a macroscopic scale of maturity stages and to changes in gonad weight. There was no evidence of pre‐ovulatory degeneration although occasional residual oocytes were present in spent ovaries. The gonadosomatic index (GSI) was high in males from October to March, and in females it increased through the winter months to a maximum immediately prior to spawning. Maximum ovarian weight was 8.3–9.0% of total body weight compared with only 0.9–1.1% for testes. These GSIs were low compared with other reports for the British Isles, suggesting lower reproductive output. Spawning occurred in the first two weeks of April at a water temperature of 6.0–7.5° C. Fecundity was described from a regression of egg number (F) on fork length (f.l.): log10F= ‐1.51 + 3.33 log10 f.l. (cm). Mean egg diameter after preservation varied from 1.1 to 2.4 mm over the size range examined. Males were mature at age 2 at a length of ≥ 27 cm, while females first spawned at age 3 at a length of ≤ 28 cm, although a minority were mature at age 2. The sex ratio was 1.2 males: 1 female at Kinord and 1.4 : 1 at Davan. Greater egg production at Davan, 50 678 ha−1, compared with Kinord, 33 557 ha−1, was due to higher individual fecundity‐at‐age resulting from better growth.Fecundity (19.29 × 103 calculated for a pike of 55 cm F.L.), relative fecundity of only 10–24 eggs per gram wet weight of adult, and GSI values were low compared with pike in North America and Eurasia. Food supply may restrict reproductive output of pike in Lochs Kinord and Davan. The abundance ratio of predator: prey is high with the main prey species, perch, present in low numbers, and the only alternative species is eel in these simple fish communities.