▁M igration ▁can ▁be ▁either ▁oblig ate , ▁meaning ▁individuals ▁must ▁migr ate , ▁or ▁fac ult ative , ▁meaning ▁individuals ▁can ▁" choose " ▁to ▁migr ate ▁or ▁not . ▁Within ▁a ▁mig rat ory ▁species ▁or ▁even ▁within ▁a ▁single ▁population , ▁often ▁not ▁all ▁individuals ▁migr ate . ▁Complete ▁migration ▁is ▁when ▁all ▁individuals ▁migr ate , ▁partial ▁migration ▁is ▁when ▁some ▁individuals ▁migr ate ▁while ▁others ▁do ▁not , ▁and ▁differential ▁migration ▁is ▁when ▁the ▁difference ▁between ▁mig rat ory ▁and ▁non - m ig rat ory ▁individuals ▁is ▁based ▁on ▁age ▁or ▁sex ▁( for ▁example ). ▁ ▁While ▁most ▁mig rat ory ▁movements ▁occur ▁on ▁an ▁annual ▁cycle , ▁some ▁daily ▁movements ▁are ▁also ▁referred ▁to ▁as ▁migration . ▁Many ▁aqu atic ▁animals ▁make ▁a ▁di el ▁vertical ▁migration , ▁trav elling ▁a ▁few ▁hundred ▁meters ▁up ▁and ▁down ▁the ▁water ▁column , ▁while ▁some ▁jel ly fish ▁make ▁daily ▁horizontal ▁migr ations , ▁travel ing ▁a ▁few ▁hundred ▁meters ▁across ▁a ▁lake . ▁ ▁Ir regular ▁( non - cyc lic al ) ▁migr ations ▁such ▁as ▁ir ru ptions ▁can ▁occur ▁under ▁pressure ▁of ▁fam ine , ▁over pop ulation ▁of ▁a ▁local ity , ▁or ▁some ▁more ▁obsc ure ▁influence . ▁ ▁Season al ▁migration ▁is ▁the ▁movement ▁of ▁various ▁species ▁from ▁one ▁habitat ▁to ▁another ▁during ▁the ▁year . ▁Resource ▁avail ability ▁changes ▁depending ▁on ▁season al ▁fl uct u ations , ▁which ▁influence ▁migration ▁patterns . ▁Some ▁species ▁such ▁as ▁Pacific ▁sal