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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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john f caddy
John F. Caddy


the main objectives of this paper
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.

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.

a brief historical resume
  • Prior to the 1940’s, trawl nets were fragile and trawlers generally avoided rocky bottoms.
  • With rapid technical advances, position-fixing allowed trawling close to, or on, rocky bottoms – which are often refugia for mature fish;
  • In the Mediterranean, the traditional management solution for increasing demersal fish yield has been to increase mesh size to permit escapement of small fishes through the codend.
  • However, recent studies suggest that indirect mortality is common during escapement of juveniles, and hence it is questionable if a mesh size increase is a way of increasing or conserving yield.
  • A more radical approach suggested is to avoid fishing on rocky bottom refugia, and conserve the spawning stock.

Most Beverton & Holt based calculations assume constant M in VPA and in Yield/Recruit calculations.

  • The historical reason for this assumption was the nature of North Sea fisheries in the 1940’s and earlier.
  • The effective minimal size of fish landed in the North Sea soon after WWII was close to the size at maturity, and mature fish were common in the catch.
  • Harvesting small fish was considered wasteful and was (and still is!) frowned on in many parts of the world.
  • I will attempt to show that this is a prejudice; based on inaccurate assumptions.
a comparison between two fishing areas for demersal fishes
A comparison between two fishing areas for demersal fishes
  • The North Sea is geophysically and politically distinct from the Mediterranean: lessons from there may not work here!
  • The N. Sea is an area with predominantly flat bottom, fished by multiple national fleets;
  • Trawling is possible in most areas, and sub-areas are difficult to close to fleet components;In contrast, the Mediterranean has a narrow shelf , rocky outcrops are common, and most areas of the nearshore shelf are fished by only 1 or 2 national fleets.
  • Rocky areas were formerly effectively closed to trawling, and formed natural ‘refuges’.
raising livestock on land do we harvest the adults or the juveniles
Raising livestock on land – do we harvest the adults or the juveniles?

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.

the refugium as defined for use in fisheries conservation
The ‘Refugium’ – as defined for use in fisheries conservation
  • The term ‘Refugium’ originally described a geographical area where the fauna/flora was protected from unfavourable environments over geological time.
  • As used in fisheries nowadays, a ‘refugium’ is an area of the fishing grounds where the fauna is protected from impacts of a specific type of fishing gear, preventing its use over irregular bottom; thus allowing potential spawners to take refuge there.

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.

the fishery refugium defined by anthony and caddy 1980 slightly simplified was
The fishery refugium defined by Anthony and Caddy (1980), slightly simplified, was:

‘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.

the proposed french fishery refugium in the gulf of lions from farrugio 2010
The proposed French fishery ‘Refugium’ in the Gulf of Lions (from Farrugio 2010).

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: -

us experience in the use of refugia from bellman et al 2005
US experience in the use of refugia(from Bellman et al. 2005).

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

areas (BLACK).


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).

the refugium paradigm
  • Note that in the 1970’s-80’s, despite heavy inshore trawling, recruitment of juvenile bottom fish continued without effort control or quotas (although closures during the recruitment season were apparently effective).
  • The ‘refugium paradigm’ proposes that the few fish that survive to maturity after encountering the inshore trawl fishery, move offshore and into rocky areas, and hence, these areas should be protected from intensive exploitation to ensure successful spawning.
Missing mortality rates: mean percent incidental mortalities of juvenile fishes passing through the cod end (North Sea/Baltic).

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!

an alternative the egg recruit or fecundity per recruit simulation
  • To investigate the possible role of refugia in recruitment, I will attempt to simulate fecundity at age using the reciprocal M-at-age function.
  • I will assume either the conventional assumption of constant F for older fishes, or a declining F, which is assumed to be due to migration out off the fishing grounds to a refugium.
calculating the future benefits which would result from protecting spawners
Calculating the future benefits which would result from protecting spawners
  • A useful perspective is that the capture of a mature female terminates a train of future benefits for population replenishment from her offspring (e.g. Prager et al. 1987; Caddy and Seijo 2002).
  • A comparison of lifetime reproductive contribution of adults to the next generation will be calculated (for hake) under 2 different hypotheses. These simulations use reasonable fishing mortality rates for adults and juveniles.

What vector of M to use for juvenile fishery calculations?

  • North Sea data on M-at-age was fitted quite well by the equation:
  • M(t) = A+B/t
  • I will use this model in the following yield and fecundity/recruit calculations.

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

offshore refugium)?

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.

two cases were considered in the egg recruit analysis
Two cases were considered in the Egg/recruit analysis:
  • Assuming in both cases, that M-at-age declines following the reciprocal relationship;
  • 1) Assuming a constant F at age for adults (Fmax = 0.5 )
  • 2) Assuming F declines after ages 4-5 (Fmax = 0.6 )
  • Both of these simulations imply that a high juvenile ‘loss’ due to predation is inevitable compared with the ‘constant M’ exercise which assumes a low predation rate on juveniles.

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

Calculation of total fecundity of 50,000 mature females at age; assuming a rapid migration to a refugium occurs as maturity approaches.

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+.

  • # eggs/fem. relative change Age 0+ survivors
  • a) F constant for ages 6+ : 84 (0%) 2.3 - 4.2
  • b) Sharp decline in F-at-age: 225 (+167%) 6.8 - 11.3
  • Thus, fecundity/recruit with a refugium leads to more than a doubling of recruits.
  • Although these figures suggest that the high fecundity of Merlucciusmerluccius may be close to adequate to replenish a population (Charnov’s axiom requires a 1:1 ratio of adult to offspring). We are very close to a generational breakeven point even with F -> 0.7!

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.

the implication of the enormous fecundity of hake and other marine fishes
The implication of the enormous fecundity of hake and other marine fishes

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.

factors not incorporated in the fecundity simulation
Factors not incorporated in the fecundity simulation
  • My assumptions in this scenario are very optimistic. Several factors were not considered that would have further reduced juvenile survival in a trawled population:
  • An unknown proportion of eggs spawned may be unviable or unfertilized.
  • 2) As noted, some 20-30% of surviving juveniles die incidentally to trawling, which is also not taken into account.
  • Thus, survival to the next generation in simulations using a peak F = 0.7
  • is close to, or below the 1:1 ratio considered in the axiom of Charnov (1993) for a sustainable population!
  • Survival to maturity is an improbable event with this mortality vector, management by refugium,should be given top priority.
  • (An incidental conclusion from this calculation is that the reciprocal equation for natural mortality at age brings us close to a reasonable measure of predation at age for yield and fecundity calculations).
other roles of refugia
Other roles of Refugia
  • Four objectives of refugia for fished species have been mentioned in the literature:
  • Enhancement of egg production;
  • Maintenance of genetic diversity;
  • Locations not fishing used for experimental studies.
  • -Refugia are potentially located where physical obstacles help achieve a reduction in fishing intensity.
  • (MPA’s also may be optimally located in areas representing larval sources);
five local trends in recent mediterranean harvest technologies that reduce reproductive performance
Five local ‘trends’ in recent Mediterranean harvest technologies.that reduce reproductive performance
  • The development of trawl fisheries on the continental slope to harvest large hake and deep water shrimp;
  • Trawling over broken bottom may capture sexually mature survivors of a variety of species;
  • Massive physical impacts of heavy trawling occur on the outer shelf and continental slope displace sediment down slope to damage deeper habitats;
  • Fishing over rough bottom enhances gear loss, inducing ‘ghost fishing’ by lost gear;
  • Trammel and gill nets and longlines are used on rocky bottom and take a significant proportion of mature fish. How to control effort and area fished with these gears?
the impacts of deep water trawling on the continental slope
The impacts of deep water trawling on the continental slope
  • Laursen (2012) and Puig et al. (2012) studied impacts of spanishindustrial scale trawling on the continental slope, which began in the mid-1960s;
  • Reworking of bottom topography by intensive trawling has occurred.
  • Heavy gear raises massive clouds of fine sediment (satellite);
  • 5,400 tonnes of sediment down-slope in just 136 days. This must have dramatically altered the ocean floor and harms deep sea habitats.
  • It must have eliminated offshore refugia containing epifauna and other sheltering features.
a summary how to manage a trawl fishery for juveniles
A summary: how to manage a trawl fishery for juveniles?
  • Small fish may suffer high incidental deaths from trawling.
  • Stomach analysis in the North Sea shows that large fish consume many small ones;
  • Hence the M component caused by predation must be very high, but falls after the first 2-3 years of life. The reciprocal equation for M-at-age appears to be a satisfactory description of this process.
  • The yield/recruit does not increase radically with size at first capture or with larger mesh sizes as it does under the erroneous assumption of ‘constant M-at-age’;
  • Large fish are infrequently taken in small mesh nets, either because they are less vulnerable, or because they move to areas outside those used by inshore trawlers.
  • The Mediterranean market and cuisine has long been adapted to using small fishes which command relatively high prices. This provides a socio-economic motive for juvenile fisheries.
implications of nurseries and refugia for fishery regulations
Implications of nurseries and refugia for fishery regulations.
  • Protecting nursery areas from bottom gear is more effective than further increases in trawl mesh size;
  • Seasonal closures of nurseries, or their protection by artificial anti-trawling equipment, is a partial conservation solution;
  • If instead of refugia, formal closures offshore could be used to regulate exploitation – BUT management by MPA’s involves black boxes installed on vessels to allow satellite tracking: a more complex approach than a simple gear design criterion as needed for refugia.
implications of refugia for fishery regulations
Implications of refugia for fishery regulations
  • Do not allow modified trawls for use on rocky bottoms!
  • Larger fish are also taken using trammel or gill nets in rocky areas. These gears should be used sparingly in refugia or MPA’s;
  • Further studies on larval and juvenile mortalities are desirable;
  • Specific monitoring of sexual maturity and spawning is suggested.

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.