John F. Caddy. THE ADVANTAGES MEDITERRANEAN SHELF HABITATs FOR CONSERVING DEMERSAL FISHES using THE REFUGIUM APPROACH . The main objectives of this paper. The main objective is to illustrate that refugia may play a key role in reproductive strategies of high fecundity marine species.
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The main objective is to illustrate that refugia may play a key role in reproductive strategies of high fecundity marine species.
To achieve Charnov’s objective, namely that the ratio of numbers of reproductively successful females in successive generations of a stable population should approximate to 1:1, the reciprocal equation for M-at-age provides a satisfactory solution.
Mature females are improbable events given this mortality strategy, but providethe vitalcontribution to population continuity. Priorities should be focussed on their conservation, inter alia, by refugia and gear design.
Most Beverton & Holt based calculations assume constant M in VPA and in Yield/Recruit calculations.
What would be the future of livestock rearing, if the farmers killed their adult sheep or cattle every year - or slaughtered their chickens once they had laid eggs?
Would leaving their offspring alone to fend for themselves be a long term solution?
The conservation of the reproductive components of a population is a feature of all livestock cultures, while conserving juveniles alone is based on the belief that high fecundity will take care of overfishing.
Is there a difference between a Refugium and a Marine Protected Area (MPA)?Some authors include the legal confines of an area to be protected within the definition of a refugium. To me, this risks confusing a refugium with a MPA.
‘A REFUGIUM is a protected enclave within the species’ distribution range (elsewhere subject to catastrophic mortality by fishing), or a characteristic of the harvesting method which allows a proportion of the stock to survive to maturity and replace the population as a whole’.
This is contrasted withthe definition given for a fishery refugium in the UNEP/GEF South China Sea Project which established legal confines to a refugium, with specific regulations applying inside them:
The UNEP/GEF definition of a refugium seems very similar to that for an MPA: –i.e., boundaries are set and specific management measures applied exclusively within them.
This is of course a feasible approach, but calls for more intensive at-sea regulation, control and enforcement.
Trawling areas on the shelf (green), lie above ‘refugia’ located on rocky areas of the continental slope.(However, the ‘yellow square’- a refugium, looks more like an MPA. (from Farrugio 2010). – Note that for most of the Mediterranean, the shelf is much narrower than here, with more rocky areas.
Obviously, MPA’s and Refugia are both useful, but the key aspect of a refugium is the difficulty of fishing there. Removing this difficulty abolishes the refugium!Prohibiting trawl designs for use on rocky bottom would protect refugia. (i.e., keep the original trawl design: a gear for use over flat sedimentary areas).Unlike MPAs, the refugium approach does not require at-sea or satellite surveillance to confirm adherence to legal MPA boundaries. Confining the trawls to clean bottom is the key to success, and gear design can be largely controlled by port inspection. Evidence from US applications of this regulatory approach, show that banning heavy gear on trawls does keep fleets from fishing on rough bottom areas: -
Trawling destroys cover
which juvenile fish need for protection from predation, & damages biodiversity.
Trawling should be limited
to flat bottom areas without dense epifauna/flora.
An illustration (right) of how prohibiting the use of chains on the trawl, leads to avoidance of rocky
Bottom trawls adapted for use over rocky bottom damage epifaunal patches which are important as shelter and feeding areas for demersal fishes. Conservation groups have called for restrictions on the areas trawled, and the elimination of gear modifications for use over rough bottom. The edge of the continental slope was originally covered by a coral biocenosis like that shown below (left) offering cover for fishes. This will be damaged by heavy trawls shown (right – see later).
Illustrating two consequences of trawling close to outcrops that fish species use as refugia, and also forage close to them. - Fishermen targeting the surrounding foraging arena may catch more fish, but add to the risk of losing gear; - Lost gear added to the outcrop will continue to ‘ghost fish’.
‘Ghost fishing’ – loss of gear on rocky refugia is not uncommon.(Summary of data provided by Macfayden et al. (2009) on net loss in different Atlantic localities).
Annual and ontogenic bathymetric migrations for 2 common species affect the efficiency of juvenile and adult closed areas and refugia. How to take this into account? (modified from Doumenge 1966).
In other words, not all juveniles escaping through the cod end are still available to harvest later at a larger mesh size – both incidental mortalities due to trawling, and predation, take a significant proportion of escapees – i.e., standard Y/R calculations are optimistic in several ways for fisheries aimed at juveniles!
Shown below is a comparison of ‘conventional’ assumptions of Y/R analysis with the supposed situation where predation on juveniles, and indirect mortalities of fishing, affect M, and the migration of maturing fish from the inshore trawl fishery to refugia reduces adult F.
These two approaches to calculating yield per recruit are compared, and illustrate the marked difference resulting from the different assumptions as to M-at-age.
Ligurian Sea + Gulf Lions: VPA outputs of F(size) and catch distributions:
In VPA’s, Fishing mortality from
inshore trawling often drops
with size or age:
(Is this because adults are
in part unavailable to capture,
having migrated to an
However, for gill nets and longlines,
adult fish are targetted.
Hypotheses used for mortality vectors (natural and fishing mortalities) in egg/recruit analysis – including the effects of either a refugium, or no refugium.
Interesting to compare the weights of the cohort that are caught and predated using M declining with age.As might be expected, predation on ages 1-3 considerably exceeds catches of the same ages.
Parameters used in the simulation:Mi = α + β /I (α = 0.1416; β = 1.4032)von Bertalanffy growth: (L∞ = 79.1 cm; K = 0.185)Fecundity at age: FECi = A.LiB : (A =2.55; B =3.07) ( – Cesarini 1994)Fishing mortality: (Hypothesized F vector with peak mortalities at age 4-5 of F = 0.7-0.8*). (*This is not the highest mortality a Mediterranean hake population has experienced – up to F=1.0. - see Caddy and Abella 1999).
Calculation of total annual fecundity of female survivors, assuming F is constant for mature fish. Population fecundity (based on Cesarini 1994) is expressed as # Eggs/size of maternal generation (starting with 50,000 females at Age 0+). Lifetime # eggs produced/size of maternal generation (age 0+) = 84.3
Changes in the Order of Magnitude (O.M.) of larval survival to age 0+ per female - (If 3-5% of larvae survive larval life (Data for Merlucciuscapensis in the SE Atlantic for the 30-35 day larval life are from Garavelli et al. 2012).
Roughly estimate how many 2nd generation age 0+ animals survive from a given population of maternal 0+ recruits. (For a hake species (Merlucciuscapensis) a planktonic survival of 3-5% was observed).
Calculate the change in fecundity/recruit relative to a constant adult F, and the survival of eggs through the planktonic stages to age 0+.
It is clear why fecundity is so high: larval, and juvenile mortalities are also high! Only about 10 out of 50,000 age 0+ females (~ 0.02%?) contribute to the next generation!Recall that Charnov’s axiom says that for a stable population, on average, a spawning female only needs to produce another spawning female in the next generation. This could be the basis for a new fisheries reference point.
In order to reconcile their enormous fecundity with Charnov’s principle, it is clear that marine organisms with this reproductive strategy have evolved it to overcome an extremely low probability for individual eggs to survive to maturity.
One other consequence of this common strategy is that it transmits to marine predators the trophic energy accumulated by larvae and juveniles from feeding on the plankton, including in their diet larval stages of macrocopic organisms which share the high mortalities of microscopic organisms.
One important conclusion of this study is how effectively the reciprocal equation for M-at-age brings numbers of survivors down close to the generational 1:1 ratio required by Charnov.
While discussing reference points, I want to add a concept that occurred to me last week. Could we prepare tables of fecundity/recruit for different levels of fishing mortality (with or without refugia)? If so, we might be able to prepare a Traffic Light approach for fishery managers based on Charnov’s criterion for a sustainable population – such that the survivors to reproductive age of the progeny are no fewer than the spawning population.
Thank you for your attention!For your information -This powerpoint presentation will be available on my fishery blog:‘My published work on fisheries science’- Caddy - Word Press.