A possible explanation for the dramatic decline in bird bycatch that occurred in 2004 is a decrease in the allowable number of inadvertent kills
A decrease in the allowable number of inadvertent kills: This could be a plausible explanation. If there were regulations or changes in policies that imposed stricter limits on the number of seabirds that could be incidentally caught, it might have led to a decline in mortality rates.
An increase in the number of fishermen working in the area: This is less likely to be a direct cause of a decline in seabird mortality rates unless the new fishermen were using significantly different and more bird-friendly fishing methods.