Differential phenological responses to climate among species are predicted to disrupt ecological interactions, but datasets to evaluate trophic mismatch are scarce. We compared phenological trends for species from four levels of a North Sea pelagic food web over 23 years in which Sea Surface Temperature (SST) increased significantly. We found little consistency in phenological trends between adjacent trophic levels. None of the phenological time series showed a significant relationship with SST, and there were no significant pairwise correlations between predator and prey phenologies, suggesting that trophic mismatching could be occurring or that phenology is relatively unimportant in this system. To investigate mismatch we looked at the relationship between finer resolution data on the timing of peak energy demand (mid chick-rearing) for five seabird species and modelled daily changes in length, and hence energy value, of a key prey species, 0-group lesser sandeels (hatched in the current year). For each seabird species an annual "Mismatch Index" was calculated to be the difference between date of mid chick-rearing and the date that sandeels reached a predicted mean threshold length of 55 mm. For all seabird species there was a trend in mismatch over the study period, such that although in general seabird populations on the Isle of May now breed later than they did 20 years ago, the delays have been insufficient to match changes in sandeel length. Mid chick-rearing now occurs well before 0-group sandeels reach 55 mm and hence the estimated energy value of individual 0-group fish at this critical time has reduced by 42% (black-legged kittiwakes) to 70% (European shags). Despite these marked changes there was no evidence that increased mismatch had fitness consequences in terms of reduced breeding success for any of the seabirds considered.