Introduction

Anthropogenic increases in the levels of carbon dioxide and other greenhouse gases over the next half-century are expected to result in an increase in global mean temperature of 1.5–4.5 °C (Houghton & Woodwell, 1989; Intergovernmental Panel on Climate Change, 1992). We know that temperature increases of this magnitude are certain to have a major impact on fish populations. The historical record indicates that annual temperature anomalies smaller than those projected under most global warming scenarios have had significant short-term impacts on species distribution and abundance (Murawski, 1993). What we do not know are the details concerning which populations are likely to be most affected in the future and how much temperatures will have to rise before effects become obvious. The first step in attempting to answer these questions is to identify those periods in the life cycle that are most sensitive to temperature change. In this regard, attention has focused on the thermal tolerance of fish embryos and larvae (Houde, 1989; Pepin, 1991; Blaxter, 1992). Embryos and larvae are generally assumed to be more sensitive to temperature change than older fish (Brett, 1970) and, thus, would appear to be likely candidates in the search for critical periods.

The aim of this review is to examine the literature critically to see whether embryos and larvae are, in fact, more sensitive than juvenile and adult fish. I begin with an examination of lethal levels.