Seasonal changes in food consumption, respiration rate, and body condition in a healthy captive male harbor porpoise (Phocoena phocoena) from the North Sea were recorded over 8 y. He was kept at water and air temperatures similar to those experienced by wild conspecifics. At the age of 3 y and 10 mo, the porpoise’s body length stabilized at 148 cm. Body mass, an indicator of body condition, increased to ~40 kg between the ages of 2 and 5.5 y, after which it fluctuated seasonally by 5 to 10 kg. The porpoise’s food consumption was ~1,200 to ~4,400 g/d but was generally ~2,400 g/d (nearly 7% of body mass). Based on the caloric content of the fish diet, his energy intake was 9,000 to 26,000 kJ/d; the average was ~18,000 kJ/d. Once his body length had stabilized, the porpoise’s daily mean respiration rate was 17 to 26 breaths per 5 min (3 to 5 breaths/min). Correlation analysis revealed that respiration rate and body mass declined with increasing water temperature and that respiration rate increased with increasing food consumption. When the porpoise’s body length was stable, his food consumption also decreased as the water temperature increased. If the data from the present study are representative of other male harbor porpoises, these results indicate that male harbor porpoises may need different amounts of food depending on the season and on whether they are growing or adult. Food consumption peaks in winter; thus, seasonality should be taken into account in energetics studies. Depending on food availability at sea, harbor porpoises may be more or less vulnerable to disturbances that decrease their foraging efficiency. With information from this longitudinal study, experts will be better informed on typical body condition patterns when considering the Interim Population Consequences of Disturbance (iPCoD) model. In addition, hypotheses about the effects of climate change on cetaceans’ susceptibility to disturbance, in relation to seasons and life history, can be generated.