Understanding Fish Life History: Growth, Development, and Reproduction Strategies

1. WFSC 448 – Fish EcophysiologyLife History Theory(assembled and modified from publicly available material) • Growth • Change of form (development) • Dispersal • Timing of reproduction • Size at birth or germination • Number, size and parity of offspring • Age at death; variability of same

2. Growth – for at least part of their life history, all organisms grow by assimilating energy and nutrients – final body size species-specific • Think through equivalancies in fish

3. Fish growth • Typically fish show a deterministic, asymptotic growth schedule • Overall, • Each species has a characteristic size-at-age path • There is individual variability in size-at-age • Each species can only attain a species-specific maximum size

4. Fish growth • Standard length – several periods (phases) of growth Onset of maturity Reduced growth of adults Rapid growth of young fish

5. Change of Form • Change of form - many organisms have dramatically different forms or stages in their life cycle • Again, think through equivalencies in fish • Don’t get stuck in categorical thinking

6. Dispersal • At some time in their lives, most organisms go through dispersal – enhances reproductive success. Spiders Milkweed Oviparous sharks The embryo is completely nourished by the yolk found in its egg. Viviparous sharks The embryo also lives completely off the yolk, but the fully developed pup is born alive. When sharks bear living offspring and their eggs have only a very thin shell, we speak of lecithotrophic viviparity (lŽkithos, gr. = yolk; trophŽ = food, vivipar = living offspring). When the offspring receive additional nourishment from the mother following a phase of living off the egg yolk prior to birth, one speaks of matrotrophic viviparity (m‡ter, lat. = mother). The amount of food supplied by the mother can vary, depending on the shark species.

7. Life History Strategies • Patterns of lifespan and reproduction • Different species may have different • characteristic LHS • LH variation may also present within species • ◊ due to genetic difference • ◊ due to environmental contingencies • LHSs and plasticity in LHS are crafted • by natural selection • LHS may take many forms • ◊ countergradient variation • ◊ alternative mating strategies in bluegill

8. Life History Strategies—Parity Semelparous species Mayfly Agave Iteroparous species

9. Based on what you know about evolution by natural selection, you can predict that species that semelparous species may have evolved this strategy because: A) semelparous parents produce more offspring if they invest all their resources in reproduction, compared to if they saved resources to survive until they can reproduce again B) semelparous parents produce offspring that are more likely to survive than offspring produced by iteroparous parents C) iteroparous parents are more likely to die before they can reproduce than are semelparous parents (all of these make sense)

10. Two factors influence evolution of semelparity vs iteroparity: • Survival probability of offspring • Probability that adults will survive to reproduce again • What else? Both probabilities are low in harsh or unpredictable environments, so semelparity will be favored

11. Life History Strategies—Fecundity Think to yourself and write a contrast: list a highly fecund and low fecundity species

12. Life History Strategies—Parental Investment Think to yourself and write a contrast: list a high and low investment fish species

13. Life History Strategies—Offspring Size Think to yourself and write a contrast: list a large and small offspring size fish species

14. Practice transference of concepts. • Often to really remove myself from my disciplinary specialization I often think of plants • Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce. • Other plants produce • fewer large seeds • that provide a large store • of energy that helps seedlings • become established.

15. General Relationship between Offspring Size and Number of Offspring Many Number of Offspring Few Small Large Offspring Size

16. Reproduction vs Survival (Mortality)

17. How does caring for offspring affect parental survival in kestrels? 100 Male Female 80 60 Parents surviving the following winter (%) 40 20 0 Reduced brood size Normal brood size Enlarged brood size Fig. 53-13

18. Reproductive Trade-offs: Reproduction vs Future Survival Reproduction vs Future Growth Current vs Future Reproduction

19. Annual Meadowgrass Current vs Future Reproduction Reproduction vs Future Growth

20. Variations in fish life cycles Within this basic strategy there is some variation, even across large pelagic species taken by tuna fisheries. Two well known species groups with very contrasting life histories are the tunas and sharks. Big implications for population dynamics and for resilience to fishing. 107 Sharks (generalised) Tuna (generalised) 106 Eggs/Larvae Juveniles 105 Adults 104 Numbers 103 102 101 Days Months Years

21. Why does M fluctuate? • Natural mortality varies throughout the life-cycle of a species • Size/age – fish may “out-grow” predators (e.g. range of predators of larval v juvenile v adult marlin) • Senescence processes and Reproductive stresses • Movement away from areas of high mortality • Behavioural changes (e.g. formation of schools) • Changes in ecosystem status (e.g. prey availability, habitat availability) • Changes in abundance (e.g. density-dependence influences, like cannibalism, prey limitations

22. A few, large offspring. Parental care in carrion beetles; unusual in insects. Fish analogues?