This project is part of a larger project on the ecological impacts and risks of extreme weather events on fisheries. This project will focus specifically on the ecological impacts of changes in temperature patterns and heat waves on the Baltic herring populations. In most ecosystems, climate change leads to an increase in mean temperature. In addition, climate change may shift seasonal temperature patterns and increase temperature variability, leading to an increase in the frequency, duration, and intensity of heat waves. Temperature controls many key processes in aquatic ecosystems. For example, it strongly influences the primary production of an ecosystem, which is critical for the persistence and dynamics of higher trophic levels such as benthic invertebrates and fish. Meanwhile, temperature also determines individual physiological rates, which ultimately alter the consumption, growth, reproduction, and mortality rates of individuals. Together, these processes determine the dynamics of aquatic populations and ecosystems. Understanding how temperature and climate change alter the dynamics of fish populations is crucial for the future management of fish stocks under high fishing pressure, such as Baltic herring. The aim of this project is to understand how seasonal temperature patterns drive the dynamics of Baltic herring populations and how an increase in the intensity, duration and frequency of heat waves alters these dynamics. In this project, we will incorporate temperature dependence into a physiologically structured population model for Baltic herring. This population model describes the dynamics of a herring population while tracking the body size distribution and condition of individuals in the population. Changes in body size and condition of individual fish are described by an individual energy budget model in terms of energy consumption, energy allocation and respiration, which in turn depend on body size, condition and temperature. The dynamics of the herring population and the interaction of the population with the environment in the population model arise from these individual-level dynamics. We will investigate the effect of seasonal temperature patterns and heat waves on population dynamics through simulations with the structured population model. With these simulations we aim to answer three specific questions. 1) How do seasonal temperature patterns drive the evolution of the reproductive timing of Baltic herring; 2) What is the short-term response of the density and structure of herring populations to a single heat wave of varying duration and intensity at different times of the year; 3) How does an increase in the frequency and a change in the distribution of heat waves throughout the year alter the long-term dynamics of herring populations and the fishery yield from these populations? This project will provide new insights needed to understand and predict the response of aquatic ecosystems to climate change. For example, it will shed new light on questions about the dynamics of Baltic herring, such as the evolution of spring spawning and autumn spawning herring populations. In addition, this project will help to understand the impact of climate change on important marine ecosystems.